Make manifest.json description more generic (facebook#6355)

Author: chrisself

template-typescript/public/index.html

@@ -9,8 +9,8 @@
     />
     <meta name="theme-color" content="#000000" />
     <!--
-      manifest.json provides metadata used when your web app is added to the
-      homescreen on Android. See https://developers.google.com/web/fundamentals/web-app-manifest/
+      manifest.json provides metadata used when your web app is installed on a
+      user's mobile device or desktop. See https://developers.google.com/web/fundamentals/web-app-manifest/
     -->
     <link rel="manifest" href="%PUBLIC_URL%/manifest.json" />
     <!--

template/public/index.html

@@ -9,8 +9,8 @@
     />
     <meta name="theme-color" content="#000000" />
     <!--
-      manifest.json provides metadata used when your web app is added to the
-      homescreen on Android. See https://developers.google.com/web/fundamentals/web-app-manifest/
+      manifest.json provides metadata used when your web app is installed on a
+      user's mobile device or desktop. See https://developers.google.com/web/fundamentals/web-app-manifest/
     -->
     <link rel="manifest" href="%PUBLIC_URL%/manifest.json" />
     <!--