PM / Runtime: Update document about callbacks

Support for device power domains has been introduced in
commit 9659cc06 (PM: Make
system-wide PM and runtime PM treat subsystems consistently),
also power domain callbacks will take precedence over subsystem ones
from commit 4d27e9dc

(PM: Make
power domain callbacks take precedence over subsystem ones).

So update part of "Device Runtime PM Callbacks" in
Documentation/power/runtime_pm.txt.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>