
With mutableUsers = true, we now ensure that all users and groups that were created declaratively, are updated or removed appropriately. Thus, adding a user to users.extraUsers and then removing it now causes the acoount to be removed from /etc/passwd. Thus user/group management is fully congruent except that users and groups that were created imperatively (via useradd/groupadd) are not touched. We distinguish between declarative and imperative users/groups by tracking the former in /var/lib/nixos/declarative-{groups,users}. With mutableUsers = false, you are now no longer required to specify UIDs/GIDs for all users. The handling of mutableUsers = true/false is the same code path; the only difference is that the "false" mode ignores the existing contents of /etc/{passwd,group}. The attribute ‘createUser’ is gone. It doesn't really make sense to specify users that shouldn't be created.
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.