authentication doc should be more clear on permission handling

[hugo]

The authentication doc should state clearly that permissions are "app_label.permission" - this is only grokkable from the samples. Additionally the difference between is_staff and is_superuser should be made more clear (is_staff can login to the admin, is_superuser has all permissions without explicitly assigning them). And the python 2.3 code for using user_passes_test is missing - and because it's a parameterized decorator, the python 2.3 syntax might not be immediatly obvious to newcomers.

[Adrian Holovaty]

(In [1302]) Fixed #848 -- Made auth docs more clear on permission handling.