If a configuration does not use services that depend on the
stateVersion, it does not need to be set.
This provides an incentive for services not to rely on
stateVersion, and not to burden users with this.
conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running
nix-doc-munge nixos/**/*.nix
nix-doc-munge --import nixos/**/*.nix
the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.
