[WP] - Add HTML to feature image metabox

gistfile1.aw

// http://wpsnipp.com/index.php/functions-php/add-html-to-feature-image-metabox/

add_filter( 'admin_post_thumbnail_html', 'add_featured_image_html');
function add_featured_image_html( $html ) {
    return $html .= '<p>This is some sample text that can be displayed within the feature image box.</p>';
}