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 :)

從 React 表單文件及其關於帶有 multiple 屬性的受控 元素的現有註記開始。釐清多個值和以 JSON 填充的 元素如何處理，並在制定指引時考慮貢獻者的使用情境。完成意味著表單文件對此情境的說明足夠清楚，可以避免回報的困惑。