docs: add note to custom BackTop demo (#35625)

* docs: add note to custom BackTop demo

* Apply suggestions from code review

Co-authored-by: afc163 <afc163@gmail.com>

components/back-top/demo/custom.md

@@ -10,10 +10,14 @@ title:
 
 可以自定义回到顶部按钮的样式，限制宽高：`40px * 40px`。
 
+> 注意：`BackTop` 需要一个可接受 `onClick` 事件的元素作为 `children`。 如果您直接将文本作为子项放置，则该组件将无法正常运行。
+
 ## en-US
 
 You can customize the style of the button, just note the size limit: no more than `40px * 40px`.
 
+> Note: `BackTop` expects a element could accept `onClick` propety as children. If you put a text directly as children the component will not function properly.
+
 ```tsx
 import React from 'react';
 import { BackTop } from 'antd';