Secondary questions:
Did it happen like that? With an officer asking if Wellington wanted to use his artillery against the enemy general, and the Duke declining being quoted as saying something along the lines of "It is not the business of commanders to be firing upon one another."
Was there any reason other than custom and politeness for declining to shoot in such a case?
To my own knowledge, I don't know but it would be wasteful to fire on commanders or what is thought to be a commander. Generally, artillery would be fired at formations and general areas due to a combination of poor optics (as in a Spyglass doesn't magnify that greatly to see an exact person) and inaccuracy of guns, which can change due to an innumerable number of reasons. So, when artillery is used, it is used to fire on a general formation or area in order to use accuracy by volume and ensure that the shot isn't wasted.
However, whenever a general did die or get injured, it was due to a random shot bouncing across the field. Jean Lannes got his legs swept from under him during Aspern-Essling by a random shot rolling across the field and Massena got his horse shot from under him. This is generally due to the inaccuracy and randomness of the battlefield.
I haven't heard an order to avoid or target generals though.
can you provide a reference to this incident? You say its "famous" and I dont ever recall reading about it.
Also note that Wellington demanded that his artillery not fire on opposing artillery. He went so far as to make it an order and at one pt. in this battle I think he was going to relieve a commander for doing that. He thought artillery vs artillery was extremely wasteful of artillery and was adamant about this point.
Whatever famous incident you are referring to, it might have some connection to this preference of Wellington. He might have said "its not the business of ARTILLERY to fire upon one another," possibly.