rcu-tasks: Add block comment laying out RCU Rude design

This commit adds a block comment that gives a high-level overview of
how RCU Rude grace periods progress.  It also gives an overview of the
memory ordering.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -645,8 +645,13 @@ void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
 // passing an empty function to schedule_on_each_cpu().  This approach
 // provides an asynchronous call_rcu_tasks_rude() API and batching
 // of concurrent calls to the synchronous synchronize_rcu_rude() API.
-// This sends IPIs far and wide and induces otherwise unnecessary context
-// switches on all online CPUs, whether idle or not.
+// This invokes schedule_on_each_cpu() in order to send IPIs far and wide
+// and induces otherwise unnecessary context switches on all online CPUs,
+// whether idle or not.
+//
+// Callback handling is provided by the rcu_tasks_kthread() function.
+//
+// Ordering is provided by the scheduler's context-switch code.
 
 // Empty function to allow workqueues to force a context switch.
 static void rcu_tasks_be_rude(struct work_struct *work)