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

docs/HandlingTextInput.md

@@ -18,7 +18,7 @@ as "🍕🍕🍕".
 import React, { Component } from 'react';
 import { AppRegistry, Text, TextInput, View } from 'react-native';
 
-class PizzaTranslator extends Component {
+export default class PizzaTranslator extends Component {
   constructor(props) {
     super(props);
     this.state = {text: ''};
@@ -40,6 +40,7 @@ class PizzaTranslator extends Component {
   }
 }
 
+// skip this line if using Create React Native App
 AppRegistry.registerComponent('PizzaTranslator', () => PizzaTranslator);
 ```