note about _/$

Author: yyx990803

source/api/options.md

@@ -30,6 +30,8 @@ data.a = 3
 vm.a   // -> 3
 ```
 
+Note that properties that start with `_` or `$` will not be proxied on the Vue instance because they may conflict with Vue's internal properties and API methods. You will have to access them as `vm.$data._property`.
+
 The object must be JSON-compliant (no circular references). You can use it just like an ordinary object, and it will look exactly the same when serialized with `JSON.stringify`. You can also share it between multiple Vue instances.
 
 A special case here is when using the `data` option in `Vue.extend()`. Since we don't want nested objects to be shared by all instances created from that extended constructor, we must provide a function that returns a fresh copy of the default data:
@@ -47,7 +49,7 @@ var MyComponent = Vue.extend({
 })
 ```
 
-<p class="tip">Under the hood, Vue.js attaches a hidden property `__ob__` and recursively converts the object's enumerable properties into getters and setters to enable dependency collection. Properties with keys that starts with `$` or `_` are skipped.</p>
+<p class="tip">Under the hood, Vue.js attaches a hidden property `__ob__` and recursively converts the object's enumerable properties into getters and setters to enable dependency collection.</p>
 
 ### props