Documentary photography is simply telling a story visually, without employing fiction.
Many documentary photographs has been changed in darkroom. One example is "migrant mother" by D. Lange.
Any photograph can be changed, documentary too and still is documentary. However doing so photographer push his work to slip out of photography medium, to make his work clasified as some fu**** manipulation, or illustration, if he is not skilled enough to know where is the limit. So many photographers are simple within the medium because of technicallyty of the equipment and predatorism and even do not think about any limits of photography as a medium, and just not making difference of darkroom "work" degree. Thinking about photographers, guys that are not within the medium because of F6 Nikon or Leica R8, in most cases darkroom "work" is not a problem for they know what they do, so is photograph changed or not should not be a part of the definition. Even and lanscape changed by hands or computer by unskilled photog is no more photography.
There also some other pifalls about documentary photography.
it is a spirit of photog's approach which determines the value of his afford, it his technical ability to express what he want to say. There is no room for exhibitionism or oportunusm or excersize of the equipment in documentary photography. Placing his personality over his subject photog will only damage the serious aims of documentary photography.
And Nat. Geo. photography is not in most cases documentary but rather Nature, or travel photography (Look and think about the name, again N. GEOGRAPHY). Example of documentary photography are D. Lange, Evans. Abbot, W.E. Smith (project Minamata came in the head),...
I still think that my defonition is fine.