lib.mdDoc: remove and replace with warning

lib/options.nix

@@ -400,9 +400,11 @@ rec {
   literalExample = lib.warn "lib.literalExample is deprecated, use lib.literalExpression instead, or use lib.literalMD for a non-Nix description." literalExpression;
 
   /* Transition marker for documentation that's already migrated to markdown
-     syntax. This is a no-op and no longer needed.
+     syntax. Has been a no-op for some while and been removed from nixpkgs.
+     Kept here to alert downstream users who may not be aware of the migration's
+     completion that it should be removed from modules.
   */
-  mdDoc = lib.id;
+  mdDoc = lib.warn "lib.mdDoc was removed from nixpkgs. Option descriptions are now in Markdown by default, you can remove any remaining uses of it.";
 
   /* For use in the `defaultText` and `example` option attributes. Causes the
      given MD text to be inserted verbatim in the documentation, for when

nixos/modules/services/networking/mtr-exporter.nix

@@ -4,7 +4,7 @@ let
   inherit (lib)
     maintainers types literalExpression
     escapeShellArg escapeShellArgs
-    mkEnableOption mkOption mkRemovedOptionModule mkIf 
+    mkEnableOption mkOption mkRemovedOptionModule mkIf
     mkPackageOption optionalString concatMapStrings concatStringsSep;
 
   cfg = config.services.mtr-exporter;