Image credits (caption) below featured image

Hi, I’m using this code block from Official WordPress documentation, to add featured image into a post:

add_filter( 'the_content', 'featured_image_before_content' );

function featured_image_before_content( $content ) {
    if ( is_singular('post') && has_post_thumbnail()) {
        $thumbnail = get_the_post_thumbnail();

<p><?php echo get_post(get_post_thumbnail_id())->post_excerpt; ?></p>

        $content = $thumbnail . $content;

		}

    return $content;
}

This works perfectly fine, but how do I modify the code so that the image caption (or the credits line) appears below the image?

TIA.