help: document wire protocol "handshake" protocol
There isn't a formal handshake protocol in the wire protocol. But
clients almost certainly need to perform particular actions before they
can communicate with a server optimally. So document what that is
so people understand what's going on at connection establishment time.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])