How to get postgres man pages in a devShell?

When I look at Nixos Packages search for PostgreSQL 16, I see it lists man pages in the outputs. When I nix shell nixpkgs#postgresql_16 I get the man pages (e.g. man psql).

But in this flak.nix, there’s no man pages for psql, although I get for htop:

{
  description = "Example Go development environment for Zero to Nix";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs"; # also valid: "nixpkgs"
  };

  outputs = { self, nixpkgs }:
    let
      allSystems = [ "x86_64-linux" ];

      nameValuePair = name: value: { inherit name value; };
      genAttrs = names: f: builtins.listToAttrs (map (n: nameValuePair n (f n)) names);
      forAllSystems = f: genAttrs allSystems (system: f {
        pkgs = import nixpkgs { inherit system; };
      });
    in
    {
      # Development environment output
      devShells = forAllSystems ({ pkgs }: {
        default = pkgs.mkShell {
          packages = with pkgs; [
            postgresql_16
            htop
          ];
        };
      });
    };
}

Can you reproduce this in our environment?

What’s going on? Why my devShell drops some man pages?