adopt a standard way of documenting service classes

An additional problem for new developers especially is this: all services should be instantiated with the \Drupal::service() method. Yet if you go to api.drupal.org to look up a service, nothing in the documentation tells what the name of the service's tag is. Somehow this needs to be documented in one place or the other. I recognize that it creates a redundancy and also may violate a theoretical best practice, but without this easily accessible, instantiating a given service amounts to special knowledge which runs contrary to the spirit accessible open-source software.