Show internal options in automatically generated docs

[aciceri]

Not sure if we really want to show internal options.

Relevant links:

• https://github.com/NixOS/nixpkgs/tree/nixos-unstable/pkgs/tools/nix/nixos-render-docs
• https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-options-doc/default.nix

[avnik]

Optional filtering is nearly impossible to implement, without patching nixpkgs. Is hardcoded in https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-options-doc/default.nix