Add comments about $watch behavior with a function (#1946) (#1395)

Author: vuejs-jp-bot

src/v2/api/index.md

@@ -1488,6 +1488,9 @@ updated: 2018-12-26
   // 関数
   vm.$watch(
     function () {
+      // everytime the expression `this.a + this.b` yields a different result,
+      // the handler will be called. It's as if we were watching a computed
+      // property without defining the computed property itself
       return this.a + this.b
     },
     function (newVal, oldVal) {