Dorethea Lange

My personal opinion
I believe this to be a very good 'portrait' photograph. If you didn't know the photographer or about the image itself - where and why it was taken, you would class it as a portrait and not documentary photography. I believe it's because your told that it is part of a series of documentary images that you accept it as one.
If i replicated the set up in a studio, or outside with a portable back drop and then displayed it with other portraits, what would be the discerning fact that it could be a documentary photograph, unless you made it part of a series of images that were meant to depict a time / era or an event.