Docs: Add a comment about the $title global usage in various admin files.

This should make it clear that the variable is used as part of the HTML `<title>` tag on admin screens.

Props ravipatel, hellofromTonya, sabernhardt, audrasjb, SergeyBiryukov.
Fixes #53729.

git-svn-id: https://develop.svn.wordpress.org/trunk@51475 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/network.php

@@ -49,9 +49,11 @@ if ( ! network_domain_check() && ( ! defined( 'WP_ALLOW_MULTISITE' ) || ! WP_ALL
 }
 
 if ( is_network_admin() ) {
+	// Used in the HTML title tag.
 	$title       = __( 'Network Setup' );
 	$parent_file = 'settings.php';
 } else {
+	// Used in the HTML title tag.
 	$title       = __( 'Create a Network of WordPress Sites' );
 	$parent_file = 'tools.php';
 }