Fixed #9607 -- Added documentation for the ``extra`` argument in test client...

Fixed #9607 -- Added documentation for the ``extra`` argument in test client methods. Thanks to jroes for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11173 bcc190cf-cafb-0310-a4f2-bffc1f526a37