Give complex FlatList example more motivation

Summary:
Previously, the example MyListItem never referenced the selected prop, leaving ambiguity about the intention of the demo.

By adding a concrete implementation with coloring based on the selected prop, we can see that this is a demo of a multi-select list for batch actions instead of, say, a click-to-navigate nested list.

Here is a working Snack demo for future reference:
https://snack.expo.io/BkRMRTGB-

References #14872

<!--
Thank you for sending the PR!

If you changed any code, please provide us with clear instructions on how you verified your changes work. In other words, a test plan is *required*. Bonus points for screenshots and videos!

Please read the Contribution Guidelines at https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md to learn more about contributing to React Native.

Happy contributing!
-->
Closes https://github.com/facebook/react-native/pull/14957

Differential Revision: D5892186

Pulled By: sahrens

fbshipit-source-id: c7b46b5f6eae8f36bd4bda7cbbd354cc22108b42

Libraries/Lists/FlatList.js

@@ -233,16 +233,16 @@ type DefaultProps = typeof defaultProps;
  *       renderItem={({item}) => <Text>{item.key}</Text>}
  *     />
  *
- * More complex example demonstrating `PureComponent` usage for perf optimization and avoiding bugs.
+ * More complex, multi-select example demonstrating `PureComponent` usage for perf optimization and avoiding bugs.
  *
  * - By binding the `onPressItem` handler, the props will remain `===` and `PureComponent` will
  *   prevent wasteful re-renders unless the actual `id`, `selected`, or `title` props change, even
- *   if the inner `SomeOtherWidget` has no such optimizations.
+ *   if the components rendered in `MyListItem` did not have such optimizations.
  * - By passing `extraData={this.state}` to `FlatList` we make sure `FlatList` itself will re-render
  *   when the `state.selected` changes. Without setting this prop, `FlatList` would not know it
  *   needs to re-render any items because it is also a `PureComponent` and the prop comparison will
  *   not show any changes.
- * - `keyExtractor` tells the list to use the `id`s for the react keys.
+ * - `keyExtractor` tells the list to use the `id`s for the react keys instead of the default `key` property.
  *
  *
  *     class MyListItem extends React.PureComponent {
@@ -251,16 +251,20 @@ type DefaultProps = typeof defaultProps;
  *       };
  *
  *       render() {
+ *         const textColor = this.props.selected ? "red" : "black";
  *         return (
- *           <SomeOtherWidget
- *             {...this.props}
- *             onPress={this._onPress}
- *           />
- *         )
+ *           <TouchableOpacity onPress={this._onPress}>
+ *             <View>
+ *               <Text style={{ color: textColor }}>
+ *                 {this.props.title}
+ *               </Text>
+ *             </View>
+ *           </TouchableOpacity>
+ *         );
  *       }
  *     }
  *
- *     class MyList extends React.PureComponent {
+ *     class MultiSelectList extends React.PureComponent {
  *       state = {selected: (new Map(): Map<string, boolean>)};
  *
  *       _keyExtractor = (item, index) => item.id;