This was only applicable to very specific hardware, and the only person with an apparent interest in maintaining it (me) no longer uses the hardware in question.
To build the manual, you need Nix installed on your system (no need
for NixOS). To install Nix, follow the instructions at
    https://nixos.org/nix/download.html
When you have Nix on your system, in the root directory of the project
(i.e., `nixpkgs`), run:
    nix-build nixos/release.nix -A manual.x86_64-linux
When this command successfully finishes, it will tell you where the
manual got generated.