Issue #3443868 by Linh Le, YevKo: Add documentation to arrow-navigation.js

/**

 * @file

 * Keyboard navigation component.

 */

((Drupal, once, { focusable }) => {

  /**

   * Attaches the keyboard navigation functionality.

   *

   * @type {Drupal~behavior}

   *

   * @prop {Drupal~behaviorAttach} attach

   *   Attaches the behavior to the `.admin-toolbar` element.

   */

  Drupal.behaviors.keyboardNavigation = {

    attach: (context) => {

      once('keyboard-processed', '.admin-toolbar', context).forEach(