> Should this documentation be added to os.rst, or would
> another place be better?

Definitely the current documentation of these APIs, yes.
The parameter should appear in the signature also, with then
the remark that it works Windows only.

There should also be a versionchanged markup.

I think there are also effects on stat/lstat, IIRC; if so,
they should be documented as well. If there are now differences,
ISTM that you can't find out whether a link is a directory symlink;
if so, *that* should be documented.