code: def action(name, factory, label=None, desc=None):
      
      _annotate("action", name, factory,
                label=label, desc=desc)
docs: Annotate a model's action.
      @param name: the name of the model's action.
      @type name: str or unicode
      @param factory: a factory to create actions.
      @type factory: IActionFactory
      @param label: the action label if specified.
      @type label: str or unicode or None
      @param desc: the action description if specified.
      @type desc: str or unicode or None