Fixed #17841 -- Clarified caching note about authentication backends. Thanks...

Fixed #17841 -- Clarified caching note about authentication backends. Thanks auzigog for the proposal and lukegb for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17752 bcc190cf-cafb-0310-a4f2-bffc1f526a37