make hs-v3 client auth configuration clearer in man page

Re: convo w/ asn yesterday on IRC (tor-dev).

Jumping off of legacy/trac#27547 (moved), this is my attempt at clearing up some things I thought should be made more explicit in the man page, such as not needing to set HiddenServiceAuthorizeClient, ClientOnionAuthDir doesn't contain just one file literally named ".auth_private", it's "some_handle.auth_private", etc.

This depends on legacy/trac#25796 (moved) (& my patch in this comment https://trac.torproject.org/projects/tor/ticket/25796#comment:10 ), since I'm following the convention in other parts of the manpage to point to spec.torproject.org, but rend-spec-v3 is not yet on that page.