doc: rcu: Encourage use of rcu_barrier in checklist

The checklist suggests rcu_barrier_bh() for RCU-bh and similarly for
sched, however these APIs are now implemented as rcu_barrier() itself due
to the RCU consolidation. This commit therefore corrects checklist.txt
to encourage use of the underlying rcu_barrier() API.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>