Missing info about changes in StrictMode discoverable through useEffect docs

This ticket was originally reported in the React issue as a potential bug: https://github.com/facebook/react/issues/24502

After a short discussion with @gaearon I volunteered to update the docs so that once someone learns about empty dependencies array in useEffect simulating componentDidMount they won't be surprised to see effect callback run twice on mount.

先从 useEffect 文档和链接的 React issue 24502 开始，然后查看文档如何描述带有空依赖数组的 effect 在 StrictMode 下的行为。完成的标准是文档明确警告：在 StrictMode 中，effect callback 可能会在挂载时运行两次，从而避免读者感到意外。