`:doc` comments for attributes loaded into scope

[roberth]

Is your feature request related to a problem? Please describe.

(with NixOS/nixpkgs#343235)

$ nix repl
Nix 2.25.0pre20240910_9c0e968
Type :? for help.
nix-repl> :lf .
Added 18 variables.

nix-repl> :doc lib
error: value does not have documentation

nix-repl> :doc outputs.lib
Attribute lib

    … defined at /nix/store/5y3fzyiz80chjxan9qgyrdwvww8n196j-source/flake.nix:27:7

    nixpkgs.lib is a combination of the Nixpkgs library, and other attributes that are not part of the Nixpkgs library, but part of the Nixpkgs flake:

      · lib.nixosSystem for creating a NixOS system configuration
    
      · lib.nixos for other NixOS-provided functionality, such as runTest

Describe the solution you'd like

Either

• Save the loaded attribute set
• Eagerly load the docs into an extra map<string, Doc> during :lf, command line installable loading, and :l
• Make the docs part of StaticEnv or something? If that doesn't degrade performance.

Describe alternatives you've considered

Additional context

The workaround suffers from #8059, as outputs is not available when an installable (CLI argument) is loaded.

Priorities

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