Fixed #10303 -- Corrected a contradiction in the docs regarding the...

Fixed #10303 -- Corrected a contradiction in the docs regarding the capabilities of the .update() clause that was introduced by the documentation for F() expressions. Thanks to gluckj for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9846 bcc190cf-cafb-0310-a4f2-bffc1f526a37