[1.3.X] Fixed #16758 - Added a warning regarding overriding default settings;...

[1.3.X] Fixed #16758 - Added a warning regarding overriding default settings; thanks cyclops for the suggestion & Aymeric Augustin for the patch.

Backport of r17566 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17567 bcc190cf-cafb-0310-a4f2-bffc1f526a37