Update examples in docs and address version lag of CRNA

Summary:
cc hramos

Pretty sure I've hit all of the places where AppRegistry is called in CRNA-pastable examples. Let me know whether you think we need to approach the version lag differently, I figure a caveat is as natural a place to call it out as any.

If you end up finding anything else that needs tweaking before cherry picking, I'm happy to push that up here too.
Closes https://github.com/facebook/react-native/pull/13744

Differential Revision: D5071038

Pulled By: hramos

fbshipit-source-id: 4a4a6f2a73079aca627f17d75a4e4b395ecbd4a8

Libraries/Text/Text.js

@@ -57,7 +57,7 @@ const viewConfig = {
  * import React, { Component } from 'react';
  * import { AppRegistry, Text, StyleSheet } from 'react-native';
  *
- * class TextInANest extends Component {
+ * export default class TextInANest extends Component {
  *   constructor(props) {
  *     super(props);
  *     this.state = {
@@ -90,7 +90,7 @@ const viewConfig = {
  *   },
  * });
  *
- * // App registration and rendering
+ * // skip this line if using Create React Native App
  * AppRegistry.registerComponent('TextInANest', () => TextInANest);
  * ```
  */