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/ms-delete-site.php

@@ -35,8 +35,10 @@ if ( isset( $_GET['h'] ) && '' !== $_GET['h'] && false !== get_option( 'delete_b
 $blog = get_site();
 $user = wp_get_current_user();
 
+// Used in the HTML title tag.
 $title       = __( 'Delete Site' );
 $parent_file = 'tools.php';
+
 require_once ABSPATH . 'wp-admin/admin-header.php';
 
 echo '<div class="wrap">';