add link to MDN event reference in v-on (vuejs#1486)

* add link to MDN event reference in v-on

* refactor link to MDN events doc

Author: JGJP

src/v2/api/index.md

@@ -1861,7 +1861,7 @@ type: api
 
   Starting in 2.4.0+, `v-on` also supports binding to an object of event/listener pairs without an argument. Note when using the object syntax, it does not support any modifiers.
 
-  When used on a normal element, it listens to **native DOM events** only. When used on a custom element component, it also listens to **custom events** emitted on that child component.
+  When used on a normal element, it listens to [**native DOM events**](https://developer.mozilla.org/en-US/docs/Web/Events) only. When used on a custom element component, it listens to **custom events** emitted on that child component.
 
   When listening to native DOM events, the method receives the native event as the only argument. If using inline statement, the statement has access to the special `$event` property: `v-on:click="handle('ok', $event)"`.