[1.2.X] Fixed #6434 - Add clarifying note about auto_now and auto_now_add....

[1.2.X] Fixed #6434 - Add clarifying note about auto_now and auto_now_add. thank adamv for the patch.

Backport of r14923 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14924 bcc190cf-cafb-0310-a4f2-bffc1f526a37