PM: Suspend/hibernation debug documentation update (rev. 2)

Update the suspend/hibernation debugging and testing documentation to describe
the newly introduced testing facilities.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Len Brown <len.brown@intel.com>