Fixed #640 - Documented that changing order_with_respect_to requires a schema...

Fixed #640 - Documented that changing order_with_respect_to requires a schema change; thanks fcurella and poirier for the draft patches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17316 bcc190cf-cafb-0310-a4f2-bffc1f526a37