code: func payloadToSlotSize(payloadSize uint64) uint64 {
  	s := sizeOfSlotHeader + payloadSize
  	return (s + sizeOfSlotHeader - 1) &^ (sizeOfSlotHeader - 1)
  }
docs: // payloadToSlotSize calculates the total size of a slot based on its payload
  // size. The  total size is the header size, plus the payload size, plus padding
  // if necessary to make the total size a multiple of sizeOfSlotHeader.