[Documentation] Add information about `child.key` to `React.Children.map`

I just spent quite a while debugging a performance issue when reordering a large list of components, which were wrapped in React.Children.map.

The issue was the implementation of the wrapping component, which used the index as the key, which caused the entire list to re-render from scratch:

<div>
  {React.Children.map(props.children, (child, index) => (
    <div key={index} className='...'>{child}</div>
  ))}
</div>

The solution was to use child.key and pass it into the child-component, which then, in turn, made the memoization of the child components work as expected:

<div>
  {React.Children.map(props.children, (child) => (
    <div key={child.key} className='...'>{child}</div>
  ))}
</div>

I feel like this should be mentioned somewhere on the documentation page for React.Children.map, since I only figured out that this is possible at all after specifically searching for it and finding #10432.

I'm happy to update the docs in a PR if this gets the OK :)

Bắt đầu với phần React.Children.map trên trang tài liệu React API được liên kết trong issue và xem lại cuộc thảo luận liên quan trong issue #10432. Bổ sung thông tin rõ ràng về child.key và tầm quan trọng của nó khi bọc children, sau đó xác minh rằng tài liệu giải thích cách sử dụng dự kiến và yếu tố cần cân nhắc về hiệu năng.