And mention you can have either lightweight 'package' or more heavyweight 'NixOS' (module) tests. This was suggested at https://github.com/ryantm/nixpkgs-update/issues/260#issuecomment-821287971 and discussed further at https://github.com/NixOS/nixpkgs/pull/119731
Nixpkgs/doc
This directory houses the sources files for the Nixpkgs manual.
You can find the rendered documentation for Nixpkgs unstable on nixos.org.
Docs for Nixpkgs stable are also available.
If you want to contribute to the documentation, here's how to do it.
If you're only getting started with Nix, go to nixos.org/learn.