doc: Use rcu_barrier() to rate-limit RCU callbacks

doc: Use rcu_barrier() to rate-limit RCU callbacks

The checklist.rst document advises periodic synchronize_rcu() invocations
to prevent callback flooding.  However, rcu_barrier() is often a better
choice.  This commit therefore adds words to this effect.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>