[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.

Issue でリンクされているドキュメントページの React.Children.map セクションから始め、既存の API の説明と例を確認してください。要求されている child への prop 注入のシナリオに基づく、明確な React の使用例を追加し、callback の引数を説明する文言にしてください。ページで、現在の混乱を招くことなく React.Children.map を使用する方法が示されれば完了です。