docs: update readme (#560)

Co-authored-by: Anthony Fu <[email protected]>

Author: Mister-Hope

README.md

@@ -6,8 +6,6 @@ Vue 2 plugin for **Composition API**
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/vuejs/composition-api/Build%20&%20Test)](https://github.com/vuejs/composition-api/actions?query=workflow%3A%22Build+%26+Test%22)
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-
-
 English | [中文](./README.zh-CN.md) ・ [**Composition API Docs**](https://composition-api.vuejs.org/)
 
 ## Installation
@@ -181,7 +179,7 @@ a.list[1].count === 1 // true
 
 <details>
 <summary>
-⚠️ `set` workaround for adding new reactive properties
+⚠️ <code>set</code> workaround for adding new reactive properties
 </summary>
 
 > ⚠️ Warning: `set` does NOT exist in Vue 3. We provide it as a workaround here, due to the limitation of [Vue 2.x reactivity system](https://vuejs.org/v2/guide/reactivity.html#For-Objects). In Vue 2, you will need to call `set` to track new keys on an `object`(similar to `Vue.set` but for `reactive objects` created by the Composition API). In Vue 3, you can just assign them like normal objects.
@@ -197,7 +195,6 @@ const a = reactive({
 set(a, 'bar', 1)
 ```
 
-
 </details>
 
 ### Template Refs
@@ -417,6 +414,7 @@ app2.component('Bar', Bar) // equivalent to Vue.use('Bar', Bar)
 </details>
 
 ### `props`
+
 <details>
 <summary>
 ⚠️ <code>toRefs(props.foo.bar)</code> will incorrectly warn when acessing nested levels of props.
@@ -427,8 +425,8 @@ app2.component('Bar', Bar) // equivalent to Vue.use('Bar', Bar)
 defineComponent({
   setup(props) {
     const { bar } = toRefs(props.foo) // it will `warn`
-    
-    // use this instead 
+
+    // use this instead
     const { foo } = toRefs(props)
     const a = foo.value.bar
   }
@@ -437,8 +435,6 @@ defineComponent({
 
 </details>
 
-
-
 ### Missing APIs
 
 The following APIs introduced in Vue 3 are not available in this plugin.
@@ -469,6 +465,31 @@ export default {
 
 </details>
 
+### `emit` Options
+
+<details>
+<summary>
+❌ <code>emit</code> option is provided in type-level only, in order to align with Vue 3's type interface. Does NOT have actual effects on the code.
+</summary>
+
+```ts
+defineComponent({
+  emit: {
+    // has no effects
+    submit: (eventOption) => {
+      if (...) {
+        return true
+      } else {
+        console.warn('Invalid submit event payload!')
+        return false
+      }
+    }
+  }
+})
+```
+
+</details>
+
 ### Performance Impact
 
 Due the the limitation of Vue2's public API. `@vue/composition-api` inevitably introduced some extract costs. It shouldn't bother you unless in extreme environments.

README.zh-CN.md

@@ -175,10 +175,9 @@ b.list[1].count === 1 // true
 
 </details>
 
-
 <details>
 <summary>
-⚠️ `set` 添加响应式属性变通方案
+⚠️ <code>set</code> 添加响应式属性变通方案
 </summary>
 
 > ⚠️ 警告: `set` 并非 `Vue 3.0` 的一部分。由于 [Vue 2.x 响应式系统的限制](https://vuejs.org/v2/guide/reactivity.html#For-Objects)，我们在插件中提供该 API 作为添加响应式属性的一个变通方案。在 Vue 3 中，你只需要直接为属性赋值即可。
@@ -393,6 +392,28 @@ watch(
 
 </details>
 
+### `props`
+
+<details>
+<summary>
+⚠️ 当使用 <code>toRefs</code> 访问深层属性对象 （如 <code>toRefs(props.foo.bar)</code> 时将会得到不正确的警告。
+⚠️ <code>isReactive(props.foo.bar)</code> 将会返回 false。
+</summary>
+  
+```ts
+defineComponent({
+  setup(props) {
+    const { bar } = toRefs(props.foo) // it will `warn`
+
+    // use this instead
+    const { foo } = toRefs(props)
+    const a = foo.value.bar
+  }
+})
+```
+
+</details>
+
 ### 缺失的 API
 
 以下在 Vue 3 新引入的 API ，在本插件中暂不适用：
@@ -423,6 +444,31 @@ export default {
 
 </details>
 
+### `emit` 选项
+
+<details>
+<summary>
+❌ <code>emit</code> 仅因在类型定义中对齐 Vue3 的选项而提供，<b>不会</b>有任何效果。
+</summary>
+
+```ts
+defineComponent({
+  emit: {
+    // 无效
+    submit: (eventOption) => {
+      if (...) {
+        return true
+      } else {
+        console.warn('Invalid submit event payload!')
+        return false
+      }
+    }
+  }
+})
+```
+
+</details>
+
 ### 性能影响
 
 由于 Vue 2 的公共 API 的限制，`@vue/composition-api` 不可避免地引入了额外的性能开销。除非在极端情况下，否则这并不会对你造成影响。