rcu: Update documentation regarding kthread_prio cmdline parameter

Inform readers that the priority of RCU no-callback threads will also
be boosted.

Signed-off-by: Alison Chaiken <achaiken@aurora.tech>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: chaiken

Documentation/admin-guide/kernel-parameters.txt

@@ -4504,6 +4504,8 @@
 			(the least-favored priority).  Otherwise, when
 			RCU_BOOST is not set, valid values are 0-99 and
 			the default is zero (non-realtime operation).
+			When RCU_NOCB_CPU is set, also adjust the
+			priority of NOCB callback kthreads.
 
 	rcutree.rcu_nocb_gp_stride= [KNL]
 			Set the number of NOCB callback kthreads in