PM: Update device power management document

The device PM document, Documentation/power/devices.txt, is badly
outdated and requires total rework to fit the current design of the
PM framework.  Make it more up to date.
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: NRandy Dunlap <randy.dunlap@oracle.com>