Having read through the entire thread a couple of times, I find myself confused by the original proposal, and by most of the responses.
Atget's pictures (thank you, Julie) are interesting and important to all of us, as a peek through the keyhole of history.
To refer to them as "documents" today may be accurate, as it is to refer to the "Declaration of Independence" can be considered by some factions in the United States.
These pictures are scenes that Atget wanted to capture, of a society and world that he saw changing, and that would soon be unrecognisable.
These were definitely NOT snapshots. The people who we see, now long gone, were well aware that they would be seen by others, and were appropriately prepared.
But these were ordinary people, in ordinary settings. These pictures, and all pictures made in the same spirit, must be cherished and preserved and viewed.
Snapshots? Not really, at least not most of them.
Documents? I wouldn't use that term, but as a window into the ( admittedly antiquated and no longer acceptable ) norms of society, possibly;
similar use of the term, referred above, is used for "documents" which are equally
antiquated and no longer acceptable.
My thoughts, yours will probably differ.