Fixed #7609 -- Noted in the model fields reference documentation that...

Fixed #7609 -- Noted in the model fields reference documentation that `PositiveIntegerField` accepts the value 0 for backwards-compatibility reasons. Thanks to everyone involved in the resolution of this issue, including Paul Collins for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16972 bcc190cf-cafb-0310-a4f2-bffc1f526a37