Fixed #12114 -- Clarified the existence of the reverse relation _set in the...

Fixed #12114 -- Clarified the existence of the reverse relation _set in the tutorial. Thanks to panfist for the suggestion, and dwillis for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12711 bcc190cf-cafb-0310-a4f2-bffc1f526a37