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

django-nonrel/django-nonrel,Instagram/django,alex/django-old,disqus/django-old,skevy/django,jamespacileo/django-france,jamespacileo/django-france,alex/django-old,sam-tsai/django-old,Smarsh/django,disqus/django-old,bfirsh/django-old,Smarsh/django,django-nonrel/django-nonrel,django-nonrel/django-nonrel,skevy/django,disqus/django-old,Smarsh/django,Instagram/django,sam-tsai/django-old,t11e/django,sam-tsai/django-old,jamespacileo/django-france,t11e/django,t11e/django,Instagram/django,mitsuhiko/django,Smarsh/django,sam-tsai/django-old,bfirsh/django-old,bfirsh/django-old,skevy/django,mitsuhiko/django,alex/django-old,mitsuhiko/django