Dont know where you are getting this. Nixpkgs is a breeze to manage compared to apt repo. Also it does not matter if you are on nixos or non-nixos system, the only difference is that nix does not take care of services on its own. What kind of docs do you miss? Nix has its own extensive nix docs page, and for packaging you also have nixpkgs documentation page - also official and not much related to nixos itself. Also nix has quite good man pages.
Stop making a list and start with eg: Nix Pills