Added a clarification to the docs about filtering across nullable intermediate

models with a NULL entry. I'm not brilliantly happy with the description
(it's too long for such an edge case, for a start), but it gets the intent across. Refs #8025.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8141 bcc190cf-cafb-0310-a4f2-bffc1f526a37