Fixed #11884 -- Clarified documentation to explain how to activate the...

Fixed #11884 -- Clarified documentation to explain how to activate the built-in admin reference. Thanks, DmitryRisenberg and timo

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12178 bcc190cf-cafb-0310-a4f2-bffc1f526a37