The text was originally added [0] following an apparently incomplete
research on how everything plays together. In fact, Nix propagates
`outputs` to the corresponding nested derivations, and there is some
messy behavior in Nixpkgs that only seems to propagate
`meta.outputsToInstall` in `buildEnv`[1].
This change moves the hints on how to use NixOS specifics to NixOS
module documentation (which is hopefully easier to find through
search.nixos.org), describes the default behavior in Nixpkgs (updating
a the link to the source), and removes the confusing mention of
`nix-env`.
the last of them should not be there to begin with. we don't want
beginners to use `nix-env`, as this is known to run them into trouble
eventually.
[0]: https://github.com/NixOS/nixpkgs/pull/76794
[1]: 1774d07242/pkgs/build-support/buildenv/default.nix (L66)
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