FB 33239 add mutable document revision

See https://gist.github.com/mikerhodes/c022e9e44d83c2e4747e for mini spec (objective-c version)
Good comment.

I think we probably should enforce that the key is the attachment name, it's just confusing otherwise. Is it possible to override the setter in a Map subclass to make sure this is the case?

I guess this is the same in iOS @rhyshort?