Add description to require a key for each child in ViewPageAndroid

Summary:
Change the header description and example code.

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What existing problem does the pull request solve?

Clarify extra requirements for a Component.

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes https://github.com/facebook/react-native/pull/13829

Differential Revision: D5661106

Pulled By: hramos

fbshipit-source-id: 39736c05f8017009cdd637930c9f89ae6c2ee7c3

Libraries/Components/ViewPager/ViewPagerAndroid.android.js

@@ -37,7 +37,7 @@ export type ViewPagerScrollState = $Enum<{
  *
  * It is important all children are `<View>`s and not composite components.
  * You can set style properties like `padding` or `backgroundColor` for each
- * child.
+ * child. It is also important that each child have a `key` prop.
  *
  * Example:
  *
@@ -47,10 +47,10 @@ export type ViewPagerScrollState = $Enum<{
  *     <ViewPagerAndroid
  *       style={styles.viewPager}
  *       initialPage={0}>
- *       <View style={styles.pageStyle}>
+ *       <View style={styles.pageStyle} key="1">
  *         <Text>First page</Text>
  *       </View>
- *       <View style={styles.pageStyle}>
+ *       <View style={styles.pageStyle} key="2">
  *         <Text>Second page</Text>
  *       </View>
  *     </ViewPagerAndroid>