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/network/themes.php

@@ -81,6 +81,7 @@ if ( $action ) {
 				$themes = array();
 			}
 
+			// Used in the HTML title tag.
 			$title       = __( 'Update Themes' );
 			$parent_file = 'themes.php';
 
@@ -333,6 +334,7 @@ get_current_screen()->set_screen_reader_content(
 	)
 );
 
+// Used in the HTML title tag.
 $title       = __( 'Themes' );
 $parent_file = 'themes.php';