Add comment policy below comment form

functions.php

function wpbeginner_comment_text_after($arg) {
    $arg['comment_notes_after'] = "We are glad you have chosen to leave a comment. Please keep in mind that comments are moderated according to our <a href='http://www.example.com/comment-policy-page/'>comment policy</a>.";
    return $arg;
}

add_filter('comment_form_defaults', 'wpbeginner_comment_text_after');