Add description to Featured Image Metabox in WordPress

functions.php

// HTML is whatever you like
function awc_post_thumbnail_add_description( $content, $post_id ){
	$post = get_post( $post_id );
	$post_type = $post->post_type;
	if ( $post_type = 'post') {
		$content .= "<div style='background-color: #eee;padding: 1rem;'>
			            <h3><label for=\"html\"><strong>Best size: <br>900px wide by 600px tall</strong></label></h3>
			          </div>";
		return $content;
		return $post_id;
	}
}