powerd(8): Add rc.conf(5) to see also

powerd_flags is mentioned in rc.conf(5) and can be set there and pass to
powerd.

PR:		258320
MFC after:	3 days
Reported by:	Michael <michael.hmich@gmail.com>

usr.sbin/powerd/powerd.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 3, 2020
+.Dd November 6, 2021
 .Dt POWERD 8
 .Os
 .Sh NAME
@@ -143,7 +143,8 @@ The default PID file.
 .Sh SEE ALSO
 .Xr acpi 4 ,
 .Xr apm 4 ,
-.Xr cpufreq 4
+.Xr cpufreq 4 ,
+.Xr rc.conf 5
 .Sh HISTORY
 The
 .Nm