Made it explicit if you accidentally override a Field from a parent model.

This was always not working reliably (model initialization and serialization
were two of the problems). Now, it's an explicit error. Also, documented.

Fixed #10252.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9974 bcc190cf-cafb-0310-a4f2-bffc1f526a37