Use a better example for Effect hook

The following example is used in:

• https://reactjs.org/docs/hooks-overview.html#effect-hook
• https://reactjs.org/docs/hooks-effect.html

import React, { useState, useEffect } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

This example creates confusion because the example doesn't actually need useEffect to operate correctly.

If you set document.title outside of the useEffect, the app behaves exactly the same.

Suggested changes

• Use an example which would not work correctly if it wasn't placed in a useEffect. It should be something that depends on the DOM having been updated. Maybe setting the document.title to a document.getElementById or something like that. This would also be a contrived/useless example. Something practical would be better.

• Show the lifecycle of useEffect early. I was most interested in the lifecycle of when this method is called. In Dan's recent blog post on effects, this is the first thing that is covered, and it should be the same for these docs.

先從 hooks-overview.html#effect-hook 和 hooks-effect.html 中的 Effect Hook 範例開始，然後將目前的範例與參考文件中的生命週期討論進行比較。以一個能說明為什麼需要 useEffect 的範例取代令人困惑的範例，並更早介紹其生命週期；完成的標準是兩個頁面都能一致地說明其行為，並展示改進後的範例。