docs: Menu API Document Update (#1460)

* Update index.zh-CN.md

补充Menu文件overflowedIndicator说明

* Update index.en-US.md

补充Menu文件overflowedIndicator说明

Author: doublehell

components/menu/index.en-US.md

@@ -29,6 +29,7 @@
 | subMenuCloseDelay | delay time to hide submenu when mouse leave, unit: second | number | 0.1 |
 | subMenuOpenDelay | delay time to show submenu when mouse enter, unit: second | number | 0 |
 | theme | color theme of the menu | string: `light` `dark` | `light` |
+| overflowedIndicator | Customized icon when menu is collapsed | DOM | `<span>···</span>` |
 
 ### Menu Events
 

components/menu/index.zh-CN.md

@@ -28,6 +28,7 @@
 | subMenuCloseDelay | 用户鼠标离开子菜单后关闭延时，单位：秒 | number | 0.1 |
 | subMenuOpenDelay | 用户鼠标进入子菜单后开启延时，单位：秒 | number | 0 |
 | theme | 主题颜色 | string: `light` `dark` | `light` |
+| overflowedIndicator | 自定义 Menu 折叠时的图标 | DOM | `<span>···</span>` |
 
 ### Menu 事件