docs(demos/showWhen): api demo

demos/show-when/app.html

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+<ion-nav id="nav" [root]="rootPage" #content></ion-nav>

demos/show-when/index.ts

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+import {App, Page, IonicApp} from 'ionic/ionic';
+
+@App({
+  templateUrl: 'app.html'
+})
+class ApiDemoApp {
+
+  constructor() {
+    this.rootPage = InitialPage;
+  }
+}
+
+@Page({
+  templateUrl: 'main.html'
+})
+export class InitialPage {
+  constructor() {
+  }
+}
+

demos/show-when/main.html

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+<ion-navbar *navbar>
+  <ion-title>ShowWhen</ion-title>
+</ion-navbar>
+
+<ion-content padding>
+  <p show-when="ios">
+    <b>(show-when="ios")</b> This text is shown on iOS only. Click the Android tab to hide this text.
+  </p>
+<ion-content>