[Docs] Add Example For React.Children.map

For https://reactjs.org/docs/react-api.html#reactchildrenmap to help illustrate as to what it does, can we kindly add an example for something like this.

const Value = ({ value }) => <p>{value}</p>

const PropsInjector = ({ children }) => React.Children.map(children, child => {
  return React.cloneElement(child, { value: 'some_value'  });
});

const App = () => (
   <PropsInjector>
        <Value />
        <Value />
    </PropsInjector>
)

I don't know if this might be an acceptable example for React.Children.map, but I would love to hear some feedback if possible.

The current documentation doesn't have a an example on how React.Children.map can be used & it doesn't make sense for someone like me

As per docs: React.Children.map(children, function[(thisArg)])

For someone like me I got very confused function[(thisArg)] when I read this & didn't know on how I could use it.

I am more then happy to make a PR for adding this example & or any better example if someone can suggest.

从 issue 中链接的文档页面的 React.Children.map 部分开始，检查现有的 API 说明和示例。添加一个基于所请求的向 child 注入 prop 场景的清晰 React 使用示例，并使用能够解释 callback 参数的措辞；当页面演示如何使用 React.Children.map 且不再造成当前的困惑时，即可完成。