Hi,
I think that the documentation is pretty clear ("[if topdown=False] ...  the directories in dirnames have already been generated by the time dirnames itself is generated"). And such behaviour is what one would expect, since it is the result of the simplest implementation (listdir(), yield <subdir>, yield <subdir>, yeild <subdir>, yeild <dir>).

I don't think that the implementation will be changed, since it is pretty to do listdir() yourself if needed, and it would make the function slower for the common use case.

Improving the documentation is always possible, what sentence would you see added (what would make the behaviour clearer to you?) ?