Fixed #8357 -- Clarified what constitutes activity when calculating session…

Fixed #8357 -- Clarified what constitutes activity when calculating session expiry. Thanks to timo for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13113 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/http/sessions.txt

@@ -194,6 +194,10 @@ It also has these methods:
             * If ``value`` is ``None``, the session reverts to using the global
               session expiry policy.
 
+      Reading a session is not considered activity for expiration
+      purposes. Session expiration is computed from the last time the
+      session was *modified*.
+
     * ``get_expiry_age()``
 
       .. versionadded:: 1.0