Was this really something that was documented, and if so was is documented well?
The in depth answers you are looking for are found within two books:
On Killing by Dave Grossman, and The Face of Battle by John Keegan.
The short answer is less. This is because modern training techniques for armies were not invented until after World War II. Prior to this, during engagements between armies, a small percentage of soldiers were actually participating in battles. Of these participants, an even smaller percentage were actively killing the enemy. The psychological reasons for why this occurs is covered in depth in On Killing.
Keegan's book The Face of Battle provides first hand accounts of what occurred at the Battle for Agincourt in 1415, along with other historical studies. Essentially, those who were actively killing others in those battles had little to no moral compunctions against killing and participating.
After World War II, when we adapted modern training methods for soldiers, like shooting at human shaped targets on rifle ranges etc. the participation rates among soldiers in battle grew larger. In turn, the PTSD rates among soldiers returning from wars also began to increase in a near proportionate amount.
Notice: This is a large post and is a block of text.
In reading these answers I think it is time to correct some disparities and amend some errors that I feel really need to be addressed.
First I want to look at an article written by Dr. Jay Winter. If are not familiar with Jay Winter he is a highly recognized scholar, professor, and specialist on the First World War and its impact on the 20th century. He wrote an article for the Journal of Contemporary History, Vol. 35, No. 1, Special Issue: Shell-Shock (Jan.,2000), pp. 7-11.
I recommend you read the article and understand his argument if you want to gauge the origins of where we get the medical diagnosis of PTSD. First understand that PTSD is not something which is restricted to the battlefield. It can manifest itself in a variety of ways which do not include war.
As for Dr. Winter and his article I will my best to hit the highlights if you cannot find the article.
The term 'shell-shock' has never before been examined in comparative historical perspective. This is a surprising omission, since the term was invented during the war, and has served as a prism through which much of the cultural history of the 1914-18 war has been viewed.
'Shell-shock' is an essential element in representations of war developed while the conflict was going on. The term, among many others, informed a language which contemporaries used to frame our sense of the war's scale, its character, its haunting legacy. Cultural history, in one sense, is the study of narratives of meaning; any cultural history of the 1914-18 war must evaluate and locate in context the various narratives, including 'shell-shock', relating to psychological injury and traumatic remembrance during and after the conflict.
The following articles examine this complex phenomenon in two ways. The first is by locating it within medical discourse and medical practice.
The crucial question is how did physicians, physiologists, neurologists and others come to an understanding of psychological breakdown during the first world war? How much did professional discourse determine diagnostic practice and prejudice?
'Shell-shock' was a term which took on a notation which moved from the medical to the metaphysical. In one set of contexts, the term had a very specific location, documented in medical files, in asylum records and by pension boards. But it also had another life, one which, in its ambiguous quality, has received less attention in a comparative context. My central argument is that the term 'shell-shock' was a specifically Anglo-Saxon representation not solely of damaged soldiers, but more generally of central facets of the war itself. To compare the different terms used in different languages, developed both during and after the war to diagnose and describe psychological disabilities among soldiers, is to disclose some striking variations. Only by making such comparisons can we fully appreciate the richness of different national traditions and perceptions within the overall cultural history of the Great War.
Many accounts attest to the variation in the incidence of paralysis among enlisted men and neurasthenia - or what we might describe as 'nervous breakdown' or 'combat fatigue', to use the second world war notation - among their officers. This distinction is ascriptive. But it has been used time and again to describe a real difference in reactions to the terrifying conditions of combat.
Enough of Winter, hopefully you understand his argument. In short he explaining that the term itself is an invention of the modern era, but that is not to say that it did not exist prior to it labeled as such. This is an important distinction. We know that this "shell-shock" is PTSD based upon the case evidence and from what Winter describes. The medical community once they were aware of this continued to look for ways to treat this psychological ailment. I think it would be wise to look at the Lost Generation as a means understanding how this manifested itself in those who lived through the First World War. How it scared them emotionally and physically.
Now with regards to prior to the 20th Century I am going to go to the American Civil War. For this I am going to go to Sarah A. M. Ford of Kurtztown University of Pennsylvania. She wrote a piece called, "Suffering in Silence: Psychological Disorders and Soldiers in the American Civil War".
She writes the following about the diagnosis of PTSD:
The most common disorder that results from exposure to combat is called post-traumatic stress disorder or more commonly known as PTSD. According to the Diagnostic and Statistical Manual of Mental Disorders, there are several categories of symptoms for PTSD. The symptoms include the experience of actual harm or threats to be harmed physically and or emotionally, intrusive symptoms that include flashbacks, disturbing dreams or memories, negative changes in cognition, the avoidance of stimuli associated with the event and changes in arousal levels. In order for there to be a diagnosis, symptoms must be present for over a month and the level of stress has to be significant enough where everyday activities are negatively affected.
Another common and relatively novel disorder is Traumatic Brain Injury (TBI). This is a neurological disorder that inhibits cognitive functioning as a result of an injury to the head. Symptoms include moderate to severe amnesia, headaches, changes in personality and accumulating more sleep than normal.3 This disorder is becoming widely recognized and diagnosed more frequently in veterans today. Over 30 percent of all casualties in Operation Iraqi Freedom (OIF) and Enduring Freedom (EF) were associated with the head or neck area.4 As many as ten to twenty percent of OIF/EF veterans have been diagnosed with TBI.5 It is plausible to assume that Civil War soldiers, who were not provided helmets, would have suffered from TBI if they experienced in injury to the head or neck region. General Anxiety Disorder and Depression are both common psychological disorders that plague many veterans today. Soldiers who experience traumatic events, such as the death of a comrade or innocent civilians, may experience depression as a result. It is logical that countless men of the Civil War era may have suffered from depression or general anxiety disorder.
Sources:
-American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders 5th ed. (Washington, DC: 2012).
-Center for Disease Control and Prevention, “Traumatic Brain Injury” http://www.cdc.gov/concussion/signs_symptoms.html#1 (accessed March 30, 2013).
-B. Capehart and D. Bass, (2012). Review: Managing posttraumatic stress disorder in combat veterans with comorbid traumatic brain injury. Journal Of Rehabilitation Research & Development 49, no.6 (2012): 789–812.
-R. Bulson, W. Jun, and J. Hayes J, “Visual Symptomatology and Referral Patterns for Operation Iraqi Freedom and Operation Enduring Freedom Veterans with Traumatic Brain Injury,” J Rehabil Res Dev 49, no.7 (2012):1075–82. http://dx.doi.org/10.1682/JRRD.2011.02.0017 (accessed March 30, 2013).
I bring this up, because of my earlier comment for classifying PTSD and understanding it. It also brings into play TBI.
Back too the main question. She discusses the origins of PTSD dating back to Egyptian Hieroglyphics and the writings of Herodotus. Yes, Hieroglyphics can be interpreted in a variety of ways, just as we can question the scholarship of Herodotus. She points to the fact that even in the seventeenth century those people, specifically soldiers who showed traits of PTSD or "any disorder associated with depression or changes in personality was termed melancholy or nostalgia." In turn the Civil War was unique in that it was really the dawning of the Modern age of military technology which would go on to changing the landscape of war. It was decades behind in medical advancements and for those encountering PTSD they would have no way of diagnosing or recognizing it. Read the article for more information as she connects "Shell Shock" with instances prior to the First World War. I believe it to be a well written piece with great source work.
She even mentions an account of a Spartan soldier who left the front line because of his trembling and later killed himself in shame.
Source: Steve Bentley, “A Short History of PTSD: From Thermopylae to Hue Soldiers Have Always Had A Disturbing Reaction To War.” The VVA Veteran, no.1 (1991): 1.
Lastly with regards to the Civil War here is another paper written by Daniel Clake, M.A., Post-Traumatic Stress Disorder and the American Civil War.
Hopefully this has answered your question and provided you with a plethora of resources. This question of PTSD was mislabeled more than anything, because people did not understand the condition or what was going on psychologically for those suffering it. How can something be documented when it is misunderstood and tossed to the wayside? It is not until the Civil War, in my opinion, that we start seeing documentation which suggests something other than what was previously thought. This notion by and large continued through the First World War as the medical field advanced. Even today in the medical community PTSD is being studied as to how to best help people who suffer from it.