nixos/doc: set groff mdoc os registers properly

[pennae]

Description of changes

the nixpkgs site-tmac sets doc-default-operating-system and doc-volume-operating-system to Nixpkgs, which is mostly correct but somewhat confusing for NixOS manpages. luckily the previous dance was overly complicated and we can instead just change the registers differently if we detect a nixpkgs groff. this also means we can use plain .Os without hard-coding the os as detected by mandoc.

followup from #213256 (comment)

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[pennae]

we thought that mandoc inspects /etc/os-release or something like that, but it looks like the os it uses is just built into the package too: https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/tools/misc/mandoc/default.nix#L27

not sure what to do with that knowledge to be honest. groff was explicitly set to Nixpkgs because nixos is just one user of nixpkgs, mandoc went the other way.

[alyssais]

not sure what to do with that knowledge to be honest. groff was explicitly set to Nixpkgs because nixos is just one user of nixpkgs, mandoc went the other way.

@sternenseemann can we make mandoc say "Nixpkgs" too? (In the general case — these pages should still say "NixOS" ideally since they're for the NixOS tools.) (The person who originally set that doesn't seem to be active any more.)

[sternenseemann]

I'd be fine with that, sure.

[pennae]

if mdoc is changed to use OS=NixOS, how much of this PR would remain? sounds like we could (should?) drop it entirely.

[sternenseemann]

You mean Nixpkgs?

[pennae]

eh, yes. 🤦

[sternenseemann]

I'd say so! I'd also drop all the shenanigans from the mdoc files and just use the default OS.

If we are too bothered by it I'd make "nixos-versions" of mandoc and groff that are injected into the man viewers via the man-db/mandoc nixos modules (since we can't change it globally via a config file?).

[pennae]

superseded by #217705