Update Pi-hole docs for v6

[MichaIng]

What still needs to be verified is that it works with a VPN OOTB. Previously it listened on a single interface only, with v6 is answers to all local origins:

  # Pi-hole interface listening modes
  #
  # Possible values are:
  #   - "LOCAL"
  #       Allow only local requests. This setting accepts DNS queries only from hosts
  #       whose address is on a local subnet, i.e., a subnet for which an interface
  #       exists on the server. It is intended to be set as a default on installation,
  #       to allow unconfigured installations to be useful but also safe from being used
  #       for DNS amplification attacks if (accidentally) running public.
...
  listeningMode = "LOCAL"

But better to test once.

And I need to test dietpi-cloudshell, but that is a different topic.