Document lxsession-defualt-apps

lxsession-default-apps is currently undocumented and on the mailing list users say things are impossible in https://lists.ubuntu.com/archives/lubuntu-users/2015-July/009821.html and similar threads on the mailing list so that even people providing help have some misconceptions on what this can do. I think this is overdue and because most of wiki and docs has been mainance and default apps for lxsession is newer than most other lxde components it did not get documented when new documentatoin pages were quite as actively getting created as it first shipped in its current form in 14.04. This can change the default programs for many different things in one tab. One tab for autostart. And one for laptop mode and keymap. All from a GUI that users may otherwise think they have to use the command line.