Documentation should state that React preserves setState callback this context

I just recently found out that setState's callback calls with the correct this context per this line in the React source. Nowhere in the documentation is it stated that React internally calls callbacks with context. This should be made clear as many assume it doesn't, leading to unnecessary measures such as arrow functions, explicit binding, etc.

The lack of explicit documentation also creates confusion with other things that do require binding, such as traditional class methods used as event handlers.

从链接的 ReactFiberUpdateQueue.js 行开始，查看现有的 setState 回调文档。完成的标准是文档明确说明回调接收正确的 this 上下文，并将这一点与用作事件处理程序的类方法区分开来。