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.
Built from https://develop.svn.wordpress.org/trunk@51475


git-svn-id: http://core.svn.wordpress.org/trunk@51086 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/network/sites.php

@@ -17,6 +17,7 @@ if ( ! current_user_can( 'manage_sites' ) ) {
 $wp_list_table = _get_list_table( 'WP_MS_Sites_List_Table' );
 $pagenum       = $wp_list_table->get_pagenum();
 
+// Used in the HTML title tag.
 $title       = __( 'Sites' );
 $parent_file = 'sites.php';