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

Author: posva

Diff for: src/v2/api/index.md

@@ -1486,6 +1486,9 @@ type: api
   // function
   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) {