[1.3.X] Fixed #17841 -- Clarified caching note about authentication backends....

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

Backport of r17752 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17753 bcc190cf-cafb-0310-a4f2-bffc1f526a37