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/plugin-install.php

@@ -47,6 +47,7 @@ if ( $pagenum > $total_pages && $total_pages > 0 ) {
 	exit;
 }
 
+// Used in the HTML title tag.
 $title       = __( 'Add Plugins' );
 $parent_file = 'plugins.php';