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/UsingAScrollView.md

@@ -16,7 +16,7 @@ This example creates a vertical `ScrollView` with both images and text mixed tog
 import React, { Component } from 'react';
 import { AppRegistry, ScrollView, Image, Text } from 'react-native'
 
-class IScrolledDownAndWhatHappenedNextShockedMe extends Component {
+export default class IScrolledDownAndWhatHappenedNextShockedMe extends Component {
   render() {
       return (
         <ScrollView>
@@ -56,7 +56,7 @@ class IScrolledDownAndWhatHappenedNextShockedMe extends Component {
   }
 }
 
-
+// skip these lines if using Create React Native App
 AppRegistry.registerComponent(
   'IScrolledDownAndWhatHappenedNextShockedMe',
   () => IScrolledDownAndWhatHappenedNextShockedMe);