Added a comment about Ruby queue destruction

src/ruby/ext/grpc/rb_completion_queue.c

@@ -72,6 +72,10 @@ static void *grpc_rb_completion_queue_pluck_no_gil(void *param) {
 
 /* Helper function to free a completion queue. */
 void grpc_rb_completion_queue_destroy(grpc_completion_queue *cq) {
+  /* Every function that adds an event to a queue also synchronously plucks
+     that event from the queue, and holds a reference to the Ruby object that
+     holds the queue, so we only get to this point if all of those functions
+     have completed, and the queue is empty */
   grpc_completion_queue_shutdown(cq);
   grpc_completion_queue_destroy(cq);
 }