Documentation: ACPI: Document _DSC object usage for enum power state

Document the use of the _DSC object for setting desirable power state
during probe.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>