Which is not always up to date, and exposes all the available options. I found myself often need to dig into nixpkgs and add missing options myself during the time I was trying out NixOS.
And believe me, that wasn't a pleasant experience.
Not sure if it was available when you tried, but this is useful when configuring, it is processed from nixpkgs and indexes all options that are available: https://nixos.org/nixos/options.html
