Add negative example warning (#1990)

Author: afontcu

Diff for: src/v2/guide/components-custom-events.md

@@ -17,6 +17,7 @@ this.$emit('myEvent')
 则监听这个名字的 kebab-case 版本是不会有任何效果的：
 
 ```html
+<!-- Won't work -->
 <my-component v-on:my-event="doSomething"></my-component>
 ```