docs(async): edit timer

ruanyf · ruanyf · commit 21a7b3285a1f · 2019-01-07T08:37:28.000+08:00
diff --git a/docs/async/timer.md b/docs/async/timer.md
@@ -190,6 +190,7 @@ setTimeout(f, 1000) // 12
 
 ```javascript
 (function() {
+  // 每轮事件循环检查一次
   var gid = setInterval(clearAllTimeouts, 0);
 
   function clearAllTimeouts() {
