Update index.md -> #watch part (#2477) (#2032)

I think it will be more clear there.

Co-authored-by: Stepan <filthy.and.filthy@gmail.com>

Author: vuejs-jp-bot

src/v2/api/index.md

@@ -687,6 +687,7 @@ updated: 2019-07-08
         handler: 'someMethod',
         immediate: true
       },
+      // you can pass array of callbacks, they will be called one-by-one
       e: [
         'handle1',
         function handle2 (val, oldVal) { /* ... */ },