sshd: Document dhcp requirement for listenAddress (#214539)

          NOTE: this will override default listening on all local addresses and port 22.

          NOTE: setting this option won't automatically enable given ports

          in firewall configuration.

          NOTE: If the IP address is not available at boot time, the following has

          to be added to make sure sshd will wait for dhcp configuration:

          ```nix

          systemd.services.sshd = {

            wants = [ "network-online.target" ];

            after = [ "network-online.target" ];

          };

          ```

          See the following issue for details: <https://github.com/NixOS/nixpkgs/issues/105570>

        '';

      };