[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 :)

Start with the React.Children.map section on the React API documentation page linked in the issue, and review the related discussion in issue #10432. Add clear information about child.key and its relevance when wrapping children, then verify that the documentation explains the intended usage and performance consideration.