docs: update App FAQ (#52904)

* docs: update App FAQ

* Update components/app/index.zh-CN.md

Co-authored-by: Amumu <yoyo837@hotmail.com>
Signed-off-by: lijianan <574980606@qq.com>

---------

Signed-off-by: lijianan <574980606@qq.com>
Co-authored-by: Amumu <yoyo837@hotmail.com>

components/app/index.en-US.md

@@ -143,4 +143,4 @@ Common props ref：[Common props](/docs/react/common-props)
 
 ### CSS Var doesn't work inside `<App component={false}>`
 
-Make sure the App `component` is a legit React component string, so when you're turning on CSS variables, there's a container to hold the CSS class name.
+Make sure the App `component` is a valid html tag, so when you're turning on CSS variables, there's a container to hold the CSS class name. If not set, it defaults to the `div` tag. If set to `false`, no additional DOM nodes will be created, and no default styles will be provided.

components/app/index.zh-CN.md

@@ -144,4 +144,4 @@ export default () => {
 
 ### CSS Var 在 `<App component={false}>` 内不起作用
 
-请确保 App 的 `component` 是一个有效的 React 组件字符串，以便在启用 CSS 变量时，有一个容器来承载 CSS 类名。
+请确保 App 的 `component` 是一个有效的 html 标签名，以便在启用 CSS 变量时有一个容器来承载 CSS 类名。如果不设置，则默认为 `div` 标签，如果设置为 `false`，则不会创建额外的 DOM 节点，也不会提供默认样式。