Fixed #17891 -- Documented a small backwards incompatibility in the...

Fixed #17891 -- Documented a small backwards incompatibility in the `password_reset` auth view. Thanks to danfairs and lukegb.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17705 bcc190cf-cafb-0310-a4f2-bffc1f526a37