functions.php

/**
 * Shows a notice in the admin once the bulk action is completed.
 */
function my_bulk_action_admin_notice() {
	if ( ! empty( $_REQUEST['bulk_draft_posts'] ) ) {
		$drafts_count = intval( $_REQUEST['bulk_draft_posts'] );

		printf(
			'<div id="message" class="updated fade">' .
			_n( '%s post moved to drafts.', '%s posts moved to drafts.', $drafts_count, 'domain' )
			. '</div>',
			$drafts_count
		);
	}
}
add_action( 'admin_notices', 'my_bulk_action_admin_notice' );