[1.1.X] Fixed #8357 -- Clarified what constitutes activity when calculating...

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

Backport of r13113 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13114 bcc190cf-cafb-0310-a4f2-bffc1f526a37