PM / sleep: Update device PM documentation to cover direct_complete

Update the device PM documentation in devices.txt and runtime_pm.txt
to reflect the changes in the system suspend and resume handling
related to the introduction of the new power.direct_complete flag.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>