Modify the api doc of extends. (#1379)

* Since the merge logic are the same,user can use mixins instead of extends

* remove the superfluous sentence

Author: ustbhuangyi

src/v2/api/index.md

@@ -978,7 +978,7 @@ type: api
 
   Allows declaratively extending another component (could be either a plain options object or a constructor) without having to use `Vue.extend`. This is primarily intended to make it easier to extend between single file components.
 
-  This is similar to `mixins`, the difference being that the component's own options takes higher priority than the source component being extended.
+  This is similar to `mixins`.
 
 - **Example:**