[1.3.X] Fixed #16552: Noted that contrib.sessions is a requirement for the...

[1.3.X] Fixed #16552: Noted that contrib.sessions is a requirement for the admin. Backport of [16806] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16807 bcc190cf-cafb-0310-a4f2-bffc1f526a37