rcu: update documentation/comments for Lai's adoption patch

Lai's RCU-callback immediate-adoption patch changes the RCU tracing
output, so update tracing.txt.  Also update a few comments to clarify
the synchronization design.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 0de359b..643c8f6 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -774,7 +774,7 @@
 }
 
 /*
- * Move preemptable DYING RCU's callbacks to other online CPU.
+ * Move preemptable RCU's callbacks from dying CPU to other online CPU.
  */
 static void rcu_preempt_send_cbs_to_online(void)
 {