I18N: Use the actual placeholder instead of a number in translator comments if the corresponding string does not use numbered placeholders.

Add missing translator comments in `WP_Theme_Install_List_Table` and `wp_notify_postauthor()`.
Add missing commas in some translator comments.

Fixes #43523.

git-svn-id: https://develop.svn.wordpress.org/trunk@42827 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-http-streams.php

@@ -250,7 +250,7 @@ class WP_Http_Streams {
 			if ( ! $stream_handle ) {
 				return new WP_Error(
 					'http_request_failed', sprintf(
-						/* translators: 1: fopen() 2: file name */
+						/* translators: 1: fopen(), 2: file name */
 						__( 'Could not open handle for %1$s to %2$s.' ),
 						'fopen()',
 						$r['filename']