Docs: update docs for Dropdown.Button (#7071)

* add "size" api document

components/dropdown/index.en-US.md

@@ -34,6 +34,7 @@ You can get the menu list by `antd.Menu`, and set a callback function `onSelect`
 | Property         | Description           | Type     | Default       |
 |--------------|----------------|----------|--------------|
 | type        | type of the button, the same as [Button](/components/button)   | string   | 'default'           |
+| size        | size of the button, the same as [Button](/components/button)   | string   | 'default'           |
 | onClick | a callback function, the same as [Button](/components/button), which will be executed when you click the button on the left       | Function   | -           |
 | trigger       | the trigger mode which can execute the drop-down action | Array<'click'\|'hover'>   | ['hover'] |
 | overlay | the dropdown menu | [Menu](/components/menu) | -           |

components/dropdown/index.zh-CN.md

@@ -35,6 +35,7 @@ title: Dropdown
 | 参数        | 说明             | 类型               | 默认值       |
 |-------------|------------------|--------------------|--------------|
 | type        | 按钮类型，和 [Button](/components/button/) 一致 | string | 'default' |
+| size        | 按钮大小，和 [Button](/components/button/) 一致 | string | 'default' |
 | onClick     | 点击左侧按钮的回调，和 [Button](/components/button/) 一致 | Function   | - |
 | trigger     | 触发下拉的行为   | Array<'click'\|'hover'> | ['hover']        |
 | overlay     | 菜单         | [Menu](/components/menu/) | -     |