code: func (c *Client) SendManyCommentAsync(fromAccount string,
  	amounts map[btcutil.Address]btcutil.Amount, minConfirms int,
  	comment string) FutureSendManyResult {
  
  	convertedAmounts := make(map[string]float64, len(amounts))
  	for addr, amount := range amounts {
  		convertedAmounts[addr.EncodeAddress()] = amount.ToBTC()
  	}
  	cmd := btcjson.NewSendManyCmd(fromAccount, convertedAmounts,
  		&minConfirms, &comment)
  	return c.sendCmd(cmd)
  }
docs: // SendManyCommentAsync returns an instance of a type that can be used to get
  // the result of the RPC at some future time by invoking the Receive function on
  // the returned instance.
  //
  // See SendManyComment for the blocking version and more details.