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.

先從 useEffect 文件和連結的 React issue 24502 開始，然後檢視文件如何描述具有空依賴陣列的 effect 在 StrictMode 下的行為。完成的標準是文件明確警告：在 StrictMode 中，effect callback 可能會在掛載時執行兩次，讓讀者不會感到意外。