Fixed #12859 -- Clarified the documentation on using multiple tables with...

Fixed #12859 -- Clarified the documentation on using multiple tables with .update() calls. Thanks to dwillis for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12515 bcc190cf-cafb-0310-a4f2-bffc1f526a37