updated Documentation/power/devices.txt

This turned into a rewrite of Documentation/power/devices.txt:

 - Provide more of the "big picture"

 - Fixup some of the horribly ancient/obsolete description of device suspend()
   semantics; lots of text just got deleted.

 - Add a decent description of PM_EVENT_* codes, including the new PRETHAW code
   needed in some swsusp scenarios.

 - Describe the new PM factorization from Linus:
     * class suspend, current suspend, then suspend_late
     * NOT suspend_prepare, it wasn't really usable
     * resume_early, current resume, class resume.

 - Updates power/state docs to be correct, and deprecate its usage except for
   driver testing.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>