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>

Documentation/RCU/checklist.rst

@@ -190,6 +190,9 @@ over a rather long period of time, but improvements are always welcome!
 
 5.	If call_rcu() or call_srcu() is used, the callback function will
 	be called from softirq context.  In particular, it cannot block.
+	If you need the callback to block, run that code in a workqueue
+	handler scheduled from the callback.  The queue_rcu_work()
+	function does this for you in the case of call_rcu().
 
 6.	Since synchronize_rcu() can block, it cannot be called
 	from any sort of irq context.  The same rule applies