[2.1.x] Clarified permission-related docs.

Backport of 632d4861dd from master

docs/topics/auth/customizing.txt

@@ -95,7 +95,7 @@ a set of optional permission related :ref:`authorization methods
 
 The ``get_user`` method takes a ``user_id`` -- which could be a username,
 database ID or whatever, but has to be the primary key of your user object --
-and returns a user object.
+and returns a user object or ``None``.
 
 The ``authenticate`` method takes a ``request`` argument and credentials as
 keyword arguments. Most of the time, it'll just look like this::

docs/topics/auth/index.txt

@@ -44,6 +44,7 @@ of these common problems have been implemented in third-party packages:
 * Password strength checking
 * Throttling of login attempts
 * Authentication against third-parties (OAuth, for example)
+* Object-level permissions
 
 Installation
 ============