rcu/nocb: Rename and document no-CB CB kthread sleep trace event

The nocb_cb_wait() function traces a "FollowerSleep" trace_rcu_nocb_wake()
event, which never was documented and is now misleading.  This commit
therefore changes "FollowerSleep" to "CBSleep", documents this, and
updates the documentation for "Sleep" as well.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

include/trace/events/rcu.h

@@ -267,7 +267,8 @@ TRACE_EVENT_RCU(rcu_exp_funnel_lock,
  *	"WakeNotPoll": Don't wake rcuo kthread because it is polling.
  *	"DeferredWake": Carried out the "IsDeferred" wakeup.
  *	"Poll": Start of new polling cycle for rcu_nocb_poll.
- *	"Sleep": Sleep waiting for CBs for !rcu_nocb_poll.
+ *	"Sleep": Sleep waiting for GP for !rcu_nocb_poll.
+ *	"CBSleep": Sleep waiting for CBs for !rcu_nocb_poll.
  *	"WokeEmpty": rcuo kthread woke to find empty list.
  *	"WokeNonEmpty": rcuo kthread woke to find non-empty list.
  *	"WaitQueue": Enqueue partially done, timed wait for it to complete.

kernel/rcu/tree_plugin.h

@@ -1891,7 +1891,7 @@ static int rcu_nocb_gp_kthread(void *arg)
  */
 static bool nocb_cb_wait(struct rcu_data *rdp)
 {
-	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FollowerSleep"));
+	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
 	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
 				 READ_ONCE(rdp->nocb_cb_head));
 	if (smp_load_acquire(&rdp->nocb_cb_head)) { /* VVV */