docs: Mention more shell functions in stdenv

[har7an]

which were previously defined but undocumented outside of setup.sh itself. Add back-references from the shell functions to the rendered upstream docs so anyone finding these from bash sources can enjoy a prettier and more integrated learning experience.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.
• Follows the automation/AI policy.

[har7an]

This is just a preview of the changes I had in mind, but I thought I'd post it here before investing more time.

Essentially I'd like to work through all of setup.sh and extract docs into the nixpkgs manual for every shell function that doesn't start with an underscore. I'd also add links to other sections where appropriate (e.g. for debugging via the custom exit hook).

Just let me know if there's interest in this. You can merge this MR and I'll get to work in (multiple?) follow-ups.

[nixpkgs-branch-check]

The PR's base branch is set to master, but this PR causes 77557 rebuilds.
It is therefore considered a mass rebuild.
Please change the base branch to the right base branch for your changes (probably staging).