[WIP] add full descriptions to options in nix help

[whentze]

fixes #5545.

[thufschmitt]

It's a bit of a shame that we can't have links in the help pages, having these flags just be a link to the doc of their respective option would be much nicer 🤔

I think the indentation of the descriptions might need fixing, but I couldn't figure out how the fmt machinery works yet.

I'm not sure you can do that with fmt, you might have to pre-process the interpolated string manually

[nixos-discourse]

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

https://discourse.nixos.org/t/summer-of-nix-documentation-stream/20351/2

[fricklerhandwerk]

@thufschmitt We can still have links in the HTML rendering.

@whentze are you still interested in pursuing this this? Documentation of configuration options is not great right now, and your PR looks like a potential quick win, given some polishing.

[iFreilicht]

I rebased this PR and fixed the indentation issues in #8818.

[fricklerhandwerk]

Superseded by #8931