nixos/zfs: Add defaultText for 'boot.zfs.package'-option

[queezle42]

I noticed that boot.zfs.package had a default value of "zfs-user-2.0.4", which seemed very specific. Looking at the source shows that the value actually depends on config.boot.zfs.enableUnstable. This commit documents that by adding a defaultText attribute.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[queezle42]

/marvin opt-in
/status awaiting_reviewer

[marvin-mk2]

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[queezle42]

Oops, wrong Marvin state.
/status needs_reviewer

[thelegy]

LGTM

[thelegy]

/status needs_merger