The current behaviour is broken for *any* descriptor which doesn't
return itself when retrieved from the class. It just so happens that
all the *other* descriptors in the standard library work that way, so
it doesn't matter if you retrieve them directly from __dict__ or
retrieve them with getattr - you'll get the descriptor object either
way.

So we should be calling getattr first, and always taking __doc__ (if
any) from the returned object.