Small update to User Guide

[Shaun McCance <shaunm gnome org>]

Paul has just committed a patch to fix the session preferences
section in the User Guide:

http://bugzilla.gnome.org/show_bug.cgi?id=502018

This mostly removes a bunch of stuff, but it does add a small
amount of content.  2.26.2 will be released tomorrow.

--
Shaun