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.

先從所引用的 ReactFiberUpdateQueue.js 行開始，確認 setState 回呼如何接收內容，然後在 React 文件中找到 setState 回呼的文件。更新指南，說明回呼的 this 內容，並將其與用作事件處理常式的類別方法區分開來。