docs: def helper(*commands):
      """Decorate a function to be the helper function of commands.
  
      Arguments:
          commands: Names of command that should trigger this function object.
  
      ---------------------------
      Interface of helper methods:
  
          @helper('some-command')
          def help_foo(self, args):
              '''
              Arguments:
                  args: A list of arguments.
  
              Returns:
                  A string that is the help message.
              '''
              pass
      """
      def decorated_func(f):
          f.__help_targets__ = list(commands)
          return f
      return decorated_func