Add Featured Image Description in WordPress

functions.php

<?php

/**
 * Add Featured Image Description
 *
 * @param  string  $content Admin post thumbnail HTML markup.
 * @param  integer $content Post ID.
 * @return string
 */
function prefix_featured_image_description( $content, $post_id ) {

    if ( 'post' === get_post_type( $post_id ) ) :

        $content .= '<p>Displayed within the main content area of post. <br> <strong>Image Dimensions: 655 x 385px</strong></p>';

    else :

        $content .= '<p><strong>Image Dimensions: 655 x 385px</strong></p>';

    endif;

    return $content;

}
add_filter( 'admin_post_thumbnail_html', 'prefix_featured_image_description');