nixos/sshd: add comment explaining different list option types

      base = pkgs.formats.keyValue {

        mkKeyValue = lib.generators.mkKeyValueDefault { inherit mkValueString; } " ";

      };

      # OpenSSH is very inconsistent with options that can take multiple values.

      # For some of them, they can simply appear multiple times and are appended, for others the

      # values must be separated by whitespace or even commas.

      # Consult either sshd_config(5) or, as last resort, the OpehSSH source for parsing

      # the options at servconf.c:process_server_config_line_depth() to determine the right "mode"

      # for each. But fortunaly this fact is documented for most of them in the manpage.

      commaSeparated = [ "Ciphers" "KexAlgorithms" "Macs" ];

      spaceSeparated = [ "AuthorizedKeysFile" "AllowGroups" "AllowUsers" "DenyGroups" "DenyUsers" ];

    in {