Make it clear that RELAY_SENDME cells don't have a payload
tor-spec taslks about SENDME cells, but doesn't say if they have a payload or not. We should probably make this explicit: https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1560
tor-spec taslks about SENDME cells, but doesn't say if they have a payload or not. We should probably make this explicit: https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1560