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/Components/TextInput/TextInput.js

@@ -74,7 +74,7 @@ const DataDetectorTypes = [
  * import React, { Component } from 'react';
  * import { AppRegistry, TextInput } from 'react-native';
  *
- * class UselessTextInput extends Component {
+ * export default class UselessTextInput extends Component {
  *   constructor(props) {
  *     super(props);
  *     this.state = { text: 'Useless Placeholder' };
@@ -91,7 +91,7 @@ const DataDetectorTypes = [
  *   }
  * }
  *
- * // App registration and rendering
+ * // skip this line if using Create React Native App
  * AppRegistry.registerComponent('AwesomeProject', () => UselessTextInput);
  * ```
  *
@@ -117,7 +117,7 @@ const DataDetectorTypes = [
  *   }
  * }
  *
- * class UselessTextInputMultiline extends Component {
+ * export default class UselessTextInputMultiline extends Component {
  *   constructor(props) {
  *     super(props);
  *     this.state = {
@@ -145,7 +145,7 @@ const DataDetectorTypes = [
  *   }
  * }
  *
- * // App registration and rendering
+ * // skip these lines if using Create React Native App
  * AppRegistry.registerComponent(
  *  'AwesomeProject',
  *  () => UselessTextInputMultiline

Libraries/Image/Image.ios.js

@@ -41,7 +41,7 @@ const ImageViewManager = NativeModules.ImageViewManager;
  * import React, { Component } from 'react';
  * import { AppRegistry, View, Image } from 'react-native';
  *
- * class DisplayAnImage extends Component {
+ * export default class DisplayAnImage extends Component {
  *   render() {
  *     return (
  *       <View>
@@ -57,7 +57,7 @@ const ImageViewManager = NativeModules.ImageViewManager;
  *   }
  * }
  *
- * // App registration and rendering
+ * // skip this line if using Create React Native App
  * AppRegistry.registerComponent('DisplayAnImage', () => DisplayAnImage);
  * ```
  *
@@ -74,7 +74,7 @@ const ImageViewManager = NativeModules.ImageViewManager;
  *   }
  * });
  *
- * class DisplayAnImageWithStyle extends Component {
+ * export default class DisplayAnImageWithStyle extends Component {
  *   render() {
  *     return (
  *       <View>
@@ -87,7 +87,7 @@ const ImageViewManager = NativeModules.ImageViewManager;
  *   }
  * }
  *
- * // App registration and rendering
+ * // skip these lines if using Create React Native App
  * AppRegistry.registerComponent(
  *   'DisplayAnImageWithStyle',
  *   () => DisplayAnImageWithStyle
@@ -96,7 +96,7 @@ const ImageViewManager = NativeModules.ImageViewManager;
  *
  * ### GIF and WebP support on Android
  *
- * By default, GIF and WebP are not supported on Android.
+ * When building your own native code, GIF and WebP are not supported by default on Android.
  *
  * You will need to add some optional modules in `android/app/build.gradle`, depending on the needs of your app.
  *

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);
  * ```
  */