docs(module:dropdown): additional notes on nzMode of nz-menu (#6773)

Author: stygian-desolator

components/dropdown/doc/index.en-US.md

@@ -33,7 +33,9 @@ import { NzDropDownModule } from 'ng-zorro-antd/dropdown';
 
 You should use [nz-menu](/components/menu/en) in `nz-dropdown`. The menu items and dividers are also available by using `nz-menu-item` and `nz-menu-divider`.
 
-> nz-menu of nz-dropdown is unselectable by default, you can make it selectable via `<ul nz-menu nzSelectable>`.
+> Note: `[nz-menu]` of `nz-dropdown` is unselectable by default, you can make it selectable via `<ul nz-menu nzSelectable>`.
+>
+> The `nzMode` of `[nz-menu]` inside `nz-dropdown-menu` can only be the default value `vertical`.
 
 ### nz-dropdown-menu
 

components/dropdown/doc/index.zh-CN.md

@@ -34,7 +34,9 @@ import { NzDropDownModule } from 'ng-zorro-antd/dropdown';
 
 菜单使用 [nz-menu](/components/menu/zh)，还包括菜单项 `[nz-menu-item]`，分割线 `[nz-menu-divider]`。
 
-> nz-dropdown 下的 nz-menu 默认不可选中。如果需要菜单可选中，可以指定 `<ul nz-menu nzSelectable>`.
+> 注意：`nz-dropdown` 下的 `[nz-menu]` 默认不可选中。如果需要菜单可选中，可以指定 `<ul nz-menu nzSelectable>`.
+>
+> `nz-dropdown-menu` 内 `[nz-menu]` 的 `nzMode` 只可以是默认值 `vertical`。
 
 ### nz-dropdown-menu