Documentation for Field Enhancers

I don't know if I have the expertise to document it myself. Here's my first crack at adding a url to the data variables for my image field. It'd not working at all though. I feel like I'm fundamentally not understanding something.

I tried doing a flat string replacement prior to this but it had really weird results where various fields inside the data were just the first letter of that string. Here I tried adding the url field and it didn't seem to add at all.

/**
 * Perform additional manipulations to image fields.
 *
 * @ResourceFieldEnhancer(
 *   id = "image_reference",
 *   label = @Translation("Image Refererence (includes url)"),
 *   description = @Translation("Includes image url")
 * )
 */
class ImageEnhancer extends ResourceFieldEnhancerBase  {


  /**
   * {@inheritdoc}
   */
  public function getOutputJsonSchema() {
    return [
      'oneOf' => [
        ['type' => 'object'],
        ['type' => 'array'],
        ['type' => 'null']
      ]
    ];
  }

  /**
   * {@inheritdoc}
   */
  protected function doUndoTransform($data, Context $context) {
    if (isset($data['url'])) {
      unset($data['url']);
    }
    return $data;
  }

  /**
   * {@inheritdoc}
   */
  protected function doTransform($data, Context $context) {
    $image = \Drupal::service('entity.repository')->loadEntityByUuid('file', $data['id']);
    $data['url'] = file_create_url($image->getFileUri());
    return $data;
  }

}

So after swapping a couple of things around I ended up with this:

  /**
   * {@inheritdoc}
   */
  protected function doUndoTransform($data, Context $context) {
    if (isset($data['meta']['url'])) {
      unset($data['meta']['url']);
    }
    return $data;
  }

  /**
   * {@inheritdoc}
   */
  protected function doTransform($data, Context $context) {
    $image = \Drupal::service('entity.repository')->loadEntityByUuid('file', $data['id']);
    $data['meta']['url'] = file_create_url($image->getFileUri());
    return $data;
  }

I don't know why it works when applying the new field to the meta sub array and not to the base. I'm also not sure why doTransform and doUndoTransform seem to be backwards but it works. Any insights as to what I'm missing here?