lib/modules: hide _module.args docs

unfortunately we can't unconditionally make this text markdown without
impacting downstream users of docs generation (as noted in NixOS#175586).
hide it entirely until the transition is complete.

lib/modules.nix

@@ -157,6 +157,11 @@ rec {
  ${if prefix == []
  then null # unset => visible
  else "internal"} = true;
+ # TODO: hidden during the markdown transition to not expose downstream
+ # users of the docs infra to markdown if they're not ready for it.
+ # we don't make this visible conditionally because it can impact
+ # performance (https://github.com/NixOS/nixpkgs/pull/208407#issuecomment-1368246192)
+ visible = false;
  # TODO: Change the type of this option to a submodule with a
  # freeformType, so that individual arguments can be documented
  # separately