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/user-new.php

@@ -245,6 +245,7 @@ Please click the following link to confirm the invite:
 	}
 }
 
+// Used in the HTML title tag.
 $title       = __( 'Add New User' );
 $parent_file = 'users.php';