doc: Tor control auth cookie file must be group readable

[kristapsk]

It is off by default, but must be enabled, unless Tor is running under the same user as JoinMarket, which is not the case except for built-in Tor (but then user does not need to configure /etc/tor/torrc).

       CookieAuthFileGroupReadable 0|1
           If this option is set to 0, don’t allow the filesystem group to read the cookie file. If the option is set to 1, make the cookie file readable
           by the default GID. [Making the file readable by other groups is not yet implemented; let us know if you need this for some reason.] (Default:
           0)