Improve documentation for <select> controlled component

In the React documentation for forms, there is only a footnote on using the controlled component with the multiple attribute. I recently spent some time getting it to work for my use case and I feel having better documentation would have sped things up. My use case involved populating the <option> elements from JSON data being returned from the server, for a select component with the multiple attribute. I am happy to contribute the documentation and raise a PR :)

Start with the React forms documentation and its existing note about controlled elements with the multiple attribute. Clarify how multiple values and JSON-populated elements are handled, and consider the contributor's use case when shaping the guidance. Done means the forms documentation explains this scenario clearly enough to avoid the reported confusion.