Fixed #12638 -- Minor documentation cleanups around the new class-based test...

Fixed #12638 -- Minor documentation cleanups around the new class-based test runners. Thanks to Ramiro Morales for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12260 bcc190cf-cafb-0310-a4f2-bffc1f526a37