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/edit.php

@@ -232,6 +232,7 @@ if ( 'wp_block' === $post_type ) {
 	wp_enqueue_style( 'wp-list-reusable-blocks' );
 }
 
+// Used in the HTML title tag.
 $title = $post_type_object->labels->name;
 
 if ( 'post' === $post_type ) {