rewrite comment.

cdc/kv/region_worker.go

@@ -379,7 +379,8 @@ func (w *regionWorker) resolveLock(ctx context.Context) error {
 	}
 }
 
-// processEvent is used as the event handle in the workerpool, controlled by the global server context.
+// processEvent is used in local region worker, also used in worker pool.
+// in the both case, should be controlled by the local region worker ctx, instead of the global server context.
 func (w *regionWorker) processEvent(event *regionStatefulEvent) error {
 	// event.state is nil when resolvedTsEvent is not nil
 	skipEvent := event.state != nil && event.state.isStale()