[1.0.X] Fixed #9607 -- Added documentation for the ``extra`` argument in test...

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

Merge of r11173 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@11174 bcc190cf-cafb-0310-a4f2-bffc1f526a37