Update docs: now you can run 8 relays per IPv4 address

I've good news for everyone looking forward to raising that limit 
further: after an investigation into the effects of allowing 4 relays 
per IP address[3] we think it is overall beneficial for the network and 
our users to raise it further to 8.

https://lists.torproject.org/pipermail/tor-relays/2023-June/021226.html