Make the new index page more useful

appengine/swarming/elements/res/imp/common/auth-signin.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   The `auth-signin` element displays sign-in/sign-out button, user email and
   avatar.
   It has a google-signin/google-signin-aware element under the hood that handles
   the actual OAuth logic.
 
   Usage:
 
     <auth-signin></auth-signin>
 
   Properties:
-    authHeaders: Object, Use this in iron-ajax to set oauth2 headers.
+    authHeaders: Object, Use this as an argument to sk.request to set oauth2 headers.
     authResponse: Object, The raw gapi.auth2.AuthResponse object.
     clientId: String, The client id to authenticate
     profile: Object, Read Only, The email address and imageurl of the logged in user.
     signedIn: Boolean, Read Only, if the user is logged in.
 
   Methods:
     signIn(): Signs the user in by popping up the authorization dialog.
     signOut(): Signs the user out.
 
   Events:
@@ skipping 97 matching lines @@
       signIn: function() {
         this.$.aware.signIn();
       },
 
       signOut: function() {
         this.$.aware.signOut();
       }
     });
   </script>
 </dom-module>