How true is the statement that US Civil War soldiers would refrain from firing on an enemy if he was...well, pooping?

For the record, I did search for this question but after two euphemisms for pooping I had to stop. That alone was a learning experience.

I have read several times that due to the high incidence of diarrhea in the Civil War that is was an unwritten rule/code between both sides that they would not fire upon someone in the act of emptying their bowels.

First, how true is this? Is it one of those "picturebook war" tall tales that gets handed down to retroactively add some honor/chivalry to the war? Second, if they did actually hold fire, was it a free pass sort of thing where they looked the other way and kept going or would they take the enemy as a POW?