update docstring for `win_getpass` to reflect code changes (GH-24967)

The code was updated in
https://github.com/python/cpython/commit/0ec88b33d093db00ec68b220247681354a650f0c
but the docstring was left untouched.

=> updated the docstring to reflect the code changes
(cherry picked from commit d4222ea6b0bb3cf7d40f23b370dc9eea5f9b7004)

Co-authored-by: Jürgen Gmach <juergen.gmach@googlemail.com>