Adding notes to a custom form
You could use the preinstalled theme item_list:

  $form['notes'] = [
    '#theme' =&gt; 'item_list',
    '#title' =&gt; $this-&gt;t('A few notes.'),
    '#list_type' =&gt; 'ul',
    '#items' =&gt; ['Item i', 'Item ii', 'Item iii'],
  ];


See https://api.drupal.org/api/drupal/core%21modules%21system%21templates%21item-list.html.twig/8.5.x