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/tools.php

@@ -39,6 +39,7 @@ if ( isset( $_GET['wp-privacy-policy-guide'] ) ) {
 /** WordPress Administration Bootstrap */
 require_once __DIR__ . '/admin.php';
 
+// Used in the HTML title tag.
 $title = __( 'Tools' );
 
 get_current_screen()->add_help_tab(