Fixed #10776 -- Added metadata targets for the contrib.admin docs, and used...

Fixed #10776 -- Added metadata targets for the contrib.admin docs, and used one of those targets to clarify the SlugField docs. Thanks to ernop for the suggestion, and timo for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10564 bcc190cf-cafb-0310-a4f2-bffc1f526a37