add explanantion for data as a function

Author: yyx990803

src/api/index.md

@@ -363,7 +363,7 @@ type: api
 
 - **Type:** `Object | Function`
 
-- **Restriction:** Only accepts `Function` when used in `Vue.extend()`.
+- **Restriction:** Only accepts `Function` when used in a component definition.
 
 - **Details:**
 
@@ -373,6 +373,8 @@ type: api
 
   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`.
 
+  When defining a **component**, `data` must be declared as a function that returns the initial data object, because there will be many instances created using the same definition. If we still use a plain object for `data`, that same object will be **shared by reference** across all instance created! By providing a `data` function, every time a new instance is created, we can simply call it to return a fresh copy of the initial data.
+
   If required, a deep clone of the original object can be obtained by passing `vm.$data` through `JSON.parse(JSON.stringify(...))`.
 
 - **Example:**
@@ -532,7 +534,7 @@ type: api
 
 - **Type:** `String | HTMLElement | Function`
 
-- **Restriction:** only accepts type `Function` when used in `Vue.extend()`.
+- **Restriction:** only accepts type `Function` when used in a component definition.
 
 - **Details:**