doc: Call out queue_rcu_work() for blocking RCU callbacks

The current checklist.rst file correctly notes that RCU callbacks execute
in BH context, and cannot block.  This commit adds words advising people
needing callbacks to block to use workqueues, for example, by replacing
call_rcu() with queue_rcu_work().

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