Site icon: Polish up Site Icon on the general settings screen.

This fixes a number of issues, chief among them:
- Updates to the site title are reflected in the preview.
- Improve alt text for preview
- Make string describing site icon more succinct.
- Add inline documentation to JavaScript

Props kebbet, jorbin, swissspidy, afercia, mukesh27, alexstine, jameskoster, andraganescu.
Fixes #54370.


Built from https://develop.svn.wordpress.org/trunk@57713


git-svn-id: http://core.svn.wordpress.org/trunk@57214 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/options.php

@@ -36,6 +36,7 @@ function options_general_add_js() {
 <script type="text/javascript">
 	jQuery( function($) {
 		var $siteName = $( '#wp-admin-bar-site-name' ).children( 'a' ).first(),
+			$siteIconPreview = $('#site-icon-preview-site-title'),
 			homeURL = ( <?php echo wp_json_encode( get_home_url() ); ?> || '' ).replace( /^(https?:\/\/)?(www\.)?/, '' );
 
 		$( '#blogname' ).on( 'input', function() {
@@ -47,6 +48,7 @@ function options_general_add_js() {
 			}
 
 			$siteName.text( title );
+			$siteIconPreview.text( title );
 		});
 
 		$( 'input[name="date_format"]' ).on( 'click', function() {