Clarified and expanded documentation for Manager.use_for_related_fields.
Clarified and expanded documentation for Manager.use_for_related_fields.

This is for Manager subclasses that are default managers, but only
sometimes.  The general rule is: "don't use it." If you really need it,
read the instructions.

git-svn-id: 4f9f921b081c523744c7bf24d959a0db39629563@10057 bcc190cf-cafb-0310-a4f2-bffc1f526a37

FlaPer87/django-nonrel,FlaPer87/django-nonrel,aparo/django-nonrel,FlaPer87/django-nonrel,aparo/django-nonrel,aparo/django-nonrel