docs update required - ADMIN_MEDIA_PREFIX issue when admin app is not used

[Aljosa Mohorovic]

if django project doesn't use admin app and ADMIN_MEDIA_PREFIX is not set it defaults to /media/ and causes problem with serving of media/static files.

issue can be solved by settings ADMIN_MEDIA_PREFIX=/media/static/admin/ or something different than MEDIA_URL although it's not actually used.

tested with django-staticfiles==0.3.4 and Django==1.2.5