Update DirAuthority man entry for client begindir, no IPv6 DirPort
Tor's DirAuthority entry says: Tor will contact the authority at address:port (the DirPort) to download directory documents. If an IPv6 address is supplied, Tor will also download directory documents at the IPv6 address on the DirPort.
But no Tor instance uses the IPv6 DirPort:
- clients (>= 0.2.8) use the IPv4 ORPort,
- clients (< 0.2.8) and relays use the IPv4 DirPort.