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This option was removed because allowing (multiple) regular users to override host entries affecting the whole system opens up a huge attack vector. There seem to be very rare cases where this might be useful. Consider setting system-wide host entries using networking.hosts, provide them via the DNS server in your network, or use networking.networkmanager.appendNameservers to point your system to another (local) nameserver to set those entries.
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.