Fixed #12204 -- Corrected the use of :djadmin: links in the testing docs, plus...

Fixed #12204 -- Corrected the use of :djadmin: links in the testing docs, plus updated a lot of old-style markup in the django-admin docs. Thanks to Art_S for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11734 bcc190cf-cafb-0310-a4f2-bffc1f526a37