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/options.php

@@ -18,6 +18,7 @@
 /** WordPress Administration Bootstrap */
 require_once __DIR__ . '/admin.php';
 
+// Used in the HTML title tag.
 $title       = __( 'Settings' );
 $this_file   = 'options.php';
 $parent_file = 'options-general.php';