Is called like this since 14321ae2431e33f5ed81f1ee43eddd29c7e9d01d, but docs were still using the old option in some cases. Reported-By: Cedric Shahabi <cedric.shahabi@gmail.com>
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.