Custom summary help text

Another option is to use a hook_form_alter() implementation to add the help text to the Summary field that's provided as part of the Long text field, such as a Body field.

Here is an example:

/**
  * Implements hook_form_alter().
  */
function my_module_form_alter(&$form, FormStateInterface &$form_state, $form_id) {
  // Add help text to summary.
  if (!empty($form['body']['widget'][0]['summary'])) {
    $form['body']['widget'][0]['summary']['#description'] = t('This is my summary help text.');
  }
}