Add :doc for lambdas to repl with nix-doc

[roberth]

Implemented in Lix. Feel free to cherry-pick it. ^.^ https://gerrit.lix.systems/c/lix/+/564

Originally posted by @lf- in #3904 (comment)

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The nix-doc dependency is not ideal, unless it could be a start towards doing more Rust.
Reimplementing without rustdoc is highly non-trivial, as we'll want to preserve evaluator performance. I'd spent ~2 days on that rather fruitlessly last year, so I'd just cherry-pick.
We might want to put it behind a package.nix flag. Perhaps buildNoGc could become a more general nix_minimal.
The static build doesn't have to be minimal (and this dependency is linked, so that works), so I'd keep that one similar to the normal build, or add the combination nix_minimal_static.

If we agree to do this, it is an alternative to

• libexpr: support :doc for nixpkgs lambdas #9054
• nix repl: Provide documentation from comment when evaluating to lambda #1652

[fricklerhandwerk]

I'd sure like the minimal Nix idea, which we could eventually use for bootstrapping Nix from vanilla Linux, and then have a Nix-only build setup for Nix itself. Whether we want a very heavy build, and in which order to approach building the complexity tower, should probably be discussed in the team.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-07-08-nix-team-meeting-minutes/49099/1