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/site-users.php

@@ -195,6 +195,7 @@ if ( isset( $_GET['action'] ) && 'update-site' === $_GET['action'] ) {
 
 add_screen_option( 'per_page' );
 
+// Used in the HTML title tag.
 /* translators: %s: Site title. */
 $title = sprintf( __( 'Edit Site: %s' ), esc_html( $details->blogname ) );