rcu: Update comments to reflect softirqs vs. kthreads

We now have kthreads only for flavors of RCU that support boosting,
so update the now-misleading comments accordingly.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index a90bf3c..ecd48a2 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1478,7 +1478,8 @@
 
 /*
  * Per-CPU kernel thread that invokes RCU callbacks.  This replaces the
- * earlier RCU softirq.
+ * RCU softirq used in flavors and configurations of RCU that do not
+ * support RCU priority boosting.
  */
 static int rcu_cpu_kthread(void *arg)
 {