[1.0.X] 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.

Backport of r10057 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10060 bcc190cf-cafb-0310-a4f2-bffc1f526a37