PM / QoS: Update Documentation for the pm_qos and dev_pm_qos frameworks

Update the documentation for the recently updated pm_qos API, kernel
and user space.  Add documentation for the per-device PM QoS
(dev_pm_qos) framework API.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>