Fixed #15236 -- corrected a typo and added a versionadded directive for the...

Fixed #15236 -- corrected a typo and added a versionadded directive for the user_logged_in and user_logged_out signals. Thanks to claudep for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15444 bcc190cf-cafb-0310-a4f2-bffc1f526a37