nixos/sshd: add comment explaining different list option types

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

nixos/modules/services/networking/ssh/sshd.nix

@@ -26,6 +26,12 @@ let
       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 {