doc: rcu: Add more rationale for using rcu_read_lock_sched in checklist

This commit explains why rcu_read_lock_sched is better than using
preempt_disable.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/checklist.txt

@@ -63,7 +63,7 @@ over a rather long period of time, but improvements are always welcome!
 	pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
 	rcu_read_lock_sched(), or by the appropriate update-side lock.
 	Disabling of preemption can serve as rcu_read_lock_sched(), but
-	is less readable.
+	is less readable and prevents lockdep from detecting locking issues.
 
 	Letting RCU-protected pointers "leak" out of an RCU read-side
 	critical section is every bid as bad as letting them leak out