rcu: Make bugginess of code sample more evident

One of the code samples in whatisRCU.txt shows a bug, but someone scanning
the document quickly might mistake it for a valid use of RCU. Add some
screaming comments to help keep speed-readers on track.

Reported-by: Nathan Zimmer <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>

Documentation/RCU/whatisRCU.txt

···
 		rcu_read_lock();
 		p = rcu_dereference(head.next);
 		rcu_read_unlock();
-		x = p->address;
+		x = p->address;	/* BUG!!! */
 		rcu_read_lock();
-		y = p->data;
+		y = p->data;	/* BUG!!! */
 		rcu_read_unlock();
 
 	Holding a reference from one RCU read-side critical section