doc: submenu required key

Author: tangjinzhou

components/menu/index.en-US.md

@@ -68,12 +68,14 @@ More layouts with navigation: [Layout](/components/layout).
 | -------------- | ----------------------------------- | ------------ | ------------- | ------- |
 | popupClassName | Sub-menu class name                 | string       |               | 1.5.0   |
 | disabled       | whether sub menu is disabled or not | boolean      | false         |         |
-| key            | Unique ID of the sub menu           | string       |               |         |
+| key            | Unique ID of the sub menu, required | string       |               |         |
 | title          | title of the sub menu               | string\|slot |               |         |
 | expandIcon     | Customized expandIcon               | slot         | arrow icon    | ｜      |
 
 The children of Menu.SubMenu must be `MenuItem` or `SubMenu`.
 
+`SubMenu` must pass the key. If it is not passed, the sub-elements under the SubMenu will be rendered in advance, and some scenes cannot be effectively highlighted.
+
 ### Menu.SubMenu Events
 
 | Events Name | Description                                          | Arguments                   |

components/menu/index.zh-CN.md

@@ -69,13 +69,15 @@ cover: https://gw.alipayobjects.com/zos/alicdn/3XZcjGpvK/Menu.svg
 | -------------- | ------------------------ | ------------ | -------- | ----- |
 | popupClassName | 子菜单样式               | string       |          | 1.5.0 |
 | disabled       | 是否禁用                 | boolean      | false    |       |
-| key            | 唯一标志                 | string       |          |       |
+| key            | 唯一标志, 必填           | string       |          |       |
 | title          | 子菜单项值               | string\|slot |          |       |
 | expandIcon     | 自定义 Menu 展开收起图标 | slot         | 箭头图标 |       |
 | icon           | 菜单图标                 | slot         |          | 2.8.0 |
 
 Menu.SubMenu 的子元素必须是 `MenuItem` 或者 `SubMenu`.
 
+`SubMenu` 必须传递 key，如不传递，该 SubMenu 下子元素将提前渲染，而且部分场景无法进行有效高亮。
+
 ### SubMenu 事件
 
 | 事件名称   | 说明           | 回调参数            |