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 :)

从 React 表单文档及其关于带有 multiple 属性的受控 元素的现有注释开始。明确多个值和由 JSON 填充的 元素是如何处理的，并在制定指导时考虑贡献者的使用场景。完成意味着表单文档对这一场景的解释足够清晰，可以避免报告中的困惑。