Docs: Add a clarification to the description about what the $output parameter in the embed_html filter doc represents.

Props musus.
Fixes #37752.

Built from https://develop.svn.wordpress.org/trunk@40945


git-svn-id: http://core.svn.wordpress.org/trunk@40795 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/embed.php

@@ -476,7 +476,7 @@ JS;
 	 *
 	 * @since 4.4.0
 	 *
-	 * @param string  $output The default HTML.
+	 * @param string  $output The default iframe tag to display embedded content.
 	 * @param WP_Post $post   Current post object.
 	 * @param int     $width  Width of the response.
 	 * @param int     $height Height of the response.