Issue #3324726 by Akhil Babu, xjm, Gauravvv, joachim: #ajax 'options' property is undocumented

 *   autocomplete, as a \Symfony\Component\HttpFoundation\JsonResponse object.

 *   See the @link menu Routing topic @endlink for more information about

 *   routing.

 *

 * @section sec_query Query parameters in Ajax requests

 * If a form uses an Ajax field, all the query parameters in the current request

 * will be also added to the Ajax POST requests along with an additional

 * 'ajax_form=1' parameter (See \Drupal\Core\Render\Element\RenderElement).

 * @code

 * $settings['options']['query'] += \Drupal::request()->query->all();

 * $settings['options']['query'][FormBuilderInterface::AJAX_FORM_REQUEST] = TRUE;

 * @endcode

 *

 * Form elements of type 'managed_file' will have an additional

 * 'element_parents' query parameter in Ajax POST requests. This parameter will

 * include the name of the element and its parents as per the render array.

 * This helps to identify the position of the element in the form (See

 * \Drupal\file\Element\ManagedFile).

 * @code

 * 'options' => [

 *   'query' => [

 *     'element_parents' => implode('/', $element['#array_parents']),

 *   ],

 * ],

 * @endcode

 */

/**