docs: def help(project, task, step, variables):
      """Run a help step."""
  
      task_name = step.args or variables['task']
  
      try:
          task = project.find_task(task_name)
      except NoSuchTaskError as e:
          yield events.task_not_found(task_name, e.similarities)
          raise StopTask
  
      text = f'# {task.name}\n'
      text += '\n'
      text += task.description
      text += '\n\n'
      text += 'Variables: {}'.format(', '.join(task.variables))
  
      yield events.help_output(text)