docs: indicate that menu items require `key`s (#35474)

components/menu/index.en-US.md

@@ -29,11 +29,12 @@ ReactDOM.render(<Alert message="After version 4.20.0, we provide a simpler usage
 ```jsx
 // works when >=4.20.0, recommended ✅
 const items = [
-  { label: 'item 1' },
-  { label: 'item 2' },
+  { label: 'item 1', key: 'item-1' }, // remember to pass the key prop
+  { label: 'item 2', key: 'item-2' }, // which is required
   {
     label: 'sub menu',
-    children: [{ label: 'item 3' }],
+    key: 'submenu'
+    children: [{ label: 'item 3', key: 'submenu-item-1' }],
   },
 ];
 return <Menu items={items} />;

components/menu/index.zh-CN.md

@@ -30,11 +30,12 @@ ReactDOM.render(<Alert message="在 4.20.0 版本后，我们提供了 <Menu ite
 ```jsx
 // >=4.20.0 可用，推荐的写法 ✅
 const items = [
-  { label: '菜单项一' },
-  { label: '菜单项二' },
+  { label: '菜单项一', key: 'item-1' }, // 菜单项务必填写 key
+  { label: '菜单项二', key: 'item-2' },
   {
     label: '子菜单',
-    children: [{ label: '子菜单项' }],
+    key: 'submenu',
+    children: [{ label: '子菜单项', key: 'submenu-item-1' }],
   },
 ];
 return <Menu items={items} />;