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/revision.php

@@ -108,6 +108,7 @@ switch ( $action ) {
 		/* translators: %s: Post title. */
 		$h1             = sprintf( __( 'Compare Revisions of “%s”' ), $post_title );
 		$return_to_post = '<a href="' . $post_edit_link . '">' . __( '&larr; Go to editor' ) . '</a>';
+		// Used in the HTML title tag.
 		$title          = __( 'Revisions' );
 
 		$redirect = false;