Third argument passed to hook_menu_local_tasks_alter is undocumented

Problem/Motivation

hook_menu_local_tasks_alter is passed a third argument in \Drupal\Core\Menu\LocalTaskManager::getLocalTasks, which is currently undocumented in menu.api.php.

The hook invocation looks like (truncated):

  /**
   * {@inheritdoc}
   */
  public function getLocalTasks($route_name, $level = 0) {
    if (!isset($this->taskData[$route_name])) {
      $cacheability = new CacheableMetadata();
      $cacheability->addCacheContexts(['route']);
[...]
        $this->moduleHandler->alter('menu_local_tasks', $this->taskData[$route_name], $route_name, $cacheability);
        $this->taskData[$route_name]['cacheability'] = $cacheability;
[...]
  }

And the hook documentation shows an example as:

function hook_menu_local_tasks_alter(&$data, $route_name) {

There's a lot of value in adding cache tags and dependencies to $cacheability in a hook, so we should document it.

Proposed resolution

Document the third argument.

Remaining tasks

Review the patch.

User interface changes

None.

API changes

The API documentation for hook_menu_local_tasks_alter will change.

Data model changes

None.