Review and update rend-spec.txt
I wanted to figure out how a client knows which relays are hosting the descriptor for a specific hidden service. I started reading through rend-spec.txt and noticed it mentions both a fixed set (v0) and a changing set (v2) of HSDirs. I believe we dropped the fixed set of HSdirs a long time ago, so rend-spec.txt needs an update.
Here is what asn and arma told me on IRC:
The current scheme (v2) means that there are 6 HSDirs for each .onion. The client visiting the hidden service computes which relays are hosting the hidden service's descriptor, in the same way that the Tor publishing the hidden service descriptor computes it. It is a function of timestamp and hash-of-public key (aka .onion address).