PM / Runtime: Make documentation of runtime_idle() agree with the code

Currently the ->runtime_idle() callback is documented as having no
return value, but in fact it returns int.  Although its return value
is ignored at the PM core level, it may be used by bus type routines
executing the drivers' ->runtime_idle() callbacks.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-by: Alan Stern <stern@rowland.harvard.edu>