Update and document realtime scheduling priority in service file

Related: OS#5687
Change-Id: I1694af74eee2624709ad6cebbc4fb37320e2ef9d
diff --git a/contrib/systemd/osmo-pcu.service b/contrib/systemd/osmo-pcu.service
index 73d8b9c..94616e3 100644
--- a/contrib/systemd/osmo-pcu.service
+++ b/contrib/systemd/osmo-pcu.service
@@ -9,10 +9,11 @@
 Restart=always
 RestartSec=2
 RestartPreventExitStatus=1
-
-# The msg queues must be read fast enough
+# CPU scheduling policy:
 CPUSchedulingPolicy=rr
-CPUSchedulingPriority=1
+# For real-time scheduling policies an integer between 1 (lowest priority) and 99 (highest priority):
+CPUSchedulingPriority=11
+# See sched(7) for further details on real-time policies and priorities
 
 [Install]
 WantedBy=multi-user.target