[1.9.x] Fixed #26514 -- Documented that User.refresh_from_db() doesn't clear the permission cache.

Backport of 2c4c67af94318b15df7d9d37b936e07e8168bc73 from master

docs/topics/auth/default.txt

@@ -285,6 +285,7 @@ the ``User`` from the database. For example::
         user.has_perm('myapp.change_bar')  # False
 
         # Request new instance of User
+        # Be aware that user.refresh_from_db() won't clear the cache.
         user = get_object_or_404(User, pk=user_id)
 
         # Permission cache is repopulated from the database