Clarified the raw_id_fields documentation and added it to InlineModelAdmin...

Clarified the raw_id_fields documentation and added it to InlineModelAdmin options section. Added examples as well. Fixes #7905. Thanks Matthew Flanagan for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8176 bcc190cf-cafb-0310-a4f2-bffc1f526a37