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.

Start with the Effect Hook examples in hooks-overview.html#effect-hook and hooks-effect.html, then compare the current example with the lifecycle discussion in the referenced documentation. Replace the confusing example with one that demonstrates why useEffect is needed and introduce its lifecycle earlier; done means both pages consistently explain the behavior and show the improved example.