Fixed #14978 -- Added versionchanged directive to --adminmedia docs clarifying...

Fixed #14978 -- Added versionchanged directive to --adminmedia docs clarifying why it's not working with staticfiles (deprecation of AdminMediaHandler).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15370 bcc190cf-cafb-0310-a4f2-bffc1f526a37