Document several undocumented builtin functions

[wentasah]

Motivation

I've found that builtins.fromTOML is not documented in the Nix manual. This PR adds the documentation of this function as well as of few others.

Context

fetchTree documentation took into account comments in #6740.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[wentasah]

I've applied the suggestions. I'll try to reformulate fetchTree doc later today. The current (and incorrect) text is based on comments in #6740.

I tried to figure out what is a immutable flake ref, but a quick look into the manual and source code does not show anything. I guess that it might be either a reference to a git repo locked in flake.lock or a reference to a specific git commit. Is that correct? Any other hints?

[fricklerhandwerk]

I suggest splitting things out into separate PRs where you expect they'd need more work, simply to avoid blocking things on each other. I've seen 3 sentence PRs being open for months because they require figuring out non-trivial things, which in combination with intermittent availability of PR authors and maintainers leads to long waiting times.

[wentasah]

Good point. I've removed the fetchTree commit from this PR and will submit it later in #8509.

[wentasah]

Thank you @fricklerhandwerk. This is definitely much better than how I would be able to formulate it. I've added you to the Co-authored-by tag.

[Ericson2314]

Thanks everyone, this is very nicely done!