Fold proposals/304-socks5-extending-hs-error-codes.txt into socks-extensions.txt
We extended the socks5 spec in another direction, to handle onion service reply codes. Currently it is documented only in a proposal that is "Status: Closed". But there are no hints about these new status codes in the main Tor specs. We should fix that.
I'm thinking socks-extension.txt is the obvious spec to pick.