Quick fixup for #372501, makes the function unreachable except for in formats.nix for the time being (there are plans to restructure pkgs-lib later, we might re-expose it in a more sane place than pkgs.formats then), adds missing parameter comment.

Ensured that it still evals with nix repl:

nix-repl> (legacyPackages.x86_64-linux.formats.json { }).type
{
  _type = "option-type";
  check = «lambda check @ /nix/store/vdmlkirf3asf5j8lkhhf08x66s33hh30-source/lib/types.nix:1042:19»;
  deprecationMessage = null;
  description = "JSON value";
  descriptionClass = "conjunction";
  emptyValue = { ... };
  functor = { ... };
  getSubModules = null;
  getSubOptions = «lambda @ /nix/store/vdmlkirf3asf5j8lkhhf08x66s33hh30-source/lib/types.nix:221:25»;
  merge = «lambda merge @ /nix/store/vdmlkirf3asf5j8lkhhf08x66s33hh30-source/lib/types.nix:1044:13»;
  name = "nullOr";
  nestedTypes = { ... };
  substSubModules = «lambda substSubModules @ /nix/store/vdmlkirf3asf5j8lkhhf08x66s33hh30-source/lib/types.nix:1059:29»;
  typeMerge = «lambda defaultTypeMerge @ /nix/store/vdmlkirf3asf5j8lkhhf08x66s33hh30-source/lib/types.nix:122:10»;
}

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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