[1.0.X] Documented patterns for adding extra managers to model subclasses.

This seems to have been a source of confusion, so now we have some explicit
examples. Fixed #9676.

Backport of r10058 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10061 bcc190cf-cafb-0310-a4f2-bffc1f526a37