[1.3.X] Fixed #17390 - Added a note to topics/auth.txt regarding how to...

[1.3.X] Fixed #17390 - Added a note to topics/auth.txt regarding how to decorate class-based generic views; thanks zsiciarz for the patch.

Backport of r17564 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17565 bcc190cf-cafb-0310-a4f2-bffc1f526a37