PM / Domains: Update documentation

Commit 4d27e9dcff00a6425d779b065ec8892e4f391661 (PM: Make power
domain callbacks take precedence over subsystem ones) forgot to
update the device power management documentation to take changes
made by it into account.  Correct that mistake.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>