reword documentation on trusted users and substituters

[fricklerhandwerk]

Motivation

found this while sweeping over open PRs.
this is to make it slightly easier to scan over

This work is sponsored by Antithesis ✨

Priorities

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[Ericson2314]

Otherwise looks good

Some things I would change, but this is mainly a reflow of the text. I'll save that for later :).

[fricklerhandwerk]

@Ericson2314 feel free to make suggestions, we can fix things while they're warm if it doesn't require ripping everything open.

[Ericson2314]

I am fine just fixing later, it will be easier once we have one sentence per line too. Also @edolstra recently made a PR which this conflicts with. (That PR cleans up the C++ and allows UIDs / GIDs too.) I think it would be good to land this one and that one soon, and then overhaul the docs.

[roberth]

I think it would be helpful to categorize some of these settings as daemon settings and put them on a separate page. That establishes the right expectations and simplifies reading for non-daemon users, trusted users, users of a chroot store, and anyone who doesn't get a permission error.

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-06-22-nix-team-meeting-minutes-65/29643/1