Clarify `REASON_DONE`
I think the comment should be the same as what spec mentions: https://github.com/torproject/torspec/blob/79da008392caed38736c73d839df7aa80628b645/tor-spec.txt#L1883
This PR might improve my understanding of this "stream closing reason" as I understand that the TCP connection in question is the one between the target server and a Tor exit node. Is that correct?