Fixed #21188 -- Introduced subclasses for to-be-removed-in-django-XX warnings

Thanks Anssi Kääriäinen for the idea and Simon Charette for the
review.

django/contrib/admindocs/views.py

@@ -15,6 +15,7 @@ from django.http import Http404
 from django.core import urlresolvers
 from django.contrib.admindocs import utils
 from django.utils.decorators import method_decorator
+from django.utils.deprecation import RemovedInDjango18Warning
 from django.utils._os import upath
 from django.utils import six
 from django.utils.translation import ugettext as _
@@ -25,7 +26,7 @@ MODEL_METHODS_EXCLUDE = ('_', 'add_', 'delete', 'save', 'set_')
 
 if getattr(settings, 'ADMIN_FOR', None):
     warnings.warn('The ADMIN_FOR setting has been removed, you can remove '
-                  'this setting from your configuration.', DeprecationWarning,
+                  'this setting from your configuration.', RemovedInDjango18Warning,
                   stacklevel=2)