[Manual] Add some information about sub-domain rules

https://www.torproject.org/docs/tor-manual.html.en

HIDDEN SERVICE OPTIONS The following options are used to configure a hidden service.

We need official statement about second-level domain(subdomain) of .onion. Likely,

this one(1) and, git.xxxxxxxxx.onion are using second level domain. I know only two URLs.

1: https://tor.stackexchange.com/questions/10068/sub-domain-of-onion-is-allowed-officialy