PM: runtime: Document return values of suspend-related API functions

Document return values for device suspend and idle related API
functions.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://patch.msgid.link/20250616061212.2286741-2-sakari.ailus@linux.intel.com


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>