'Bundles' is not a required field

As entity_get_info() handles the missing bundles key for single bundles:

  // If no bundle key is provided, assume a single bundle, named after
  // the entity type.
  if (empty($entity_info[$name]['entity keys']['bundle']) && empty($entity_info[$name]['bundles'])) {
    $entity_info[$name]['bundles'] = array($name => array('label' => $entity_info[$name]['label']));
  }

we should document this behaviour at the hook_entity_info() docs.