Documentary Photography

I think it is absolutely wrong to set up a shot if you are trying to document something. If you are setting somehting up it then becomes a staged photograph. Timothy O'Sullivan was considered doing documentary photography although in his photograph Harvest of Death, the entire thing was staged!!! How is that documenting somehting?!?! It's not real, although it looks real and soldiers were really dead on the ground, he placed certain men in specific areas on the ground in order to capture the "documentary" photograph he was supposed to. That to me is not documentary photography. Documentary photography is somehting you are capturing in the heat of the moment... Am i wrong here?!?