[1.0.X] Fixed #9254 -- Added information to the porting guide about the removal

of "core" and the new methods on model file- and image-fields.

Backport of r9166 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9172 bcc190cf-cafb-0310-a4f2-bffc1f526a37