remove the misleading warning on using `nix-env` for split outputs (#255947)

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]: https://github.com/NixOS/nixpkgs/blob/1774d07242995050d2d8a91cb4da0855eac2e472/pkgs/build-support/buildenv/default.nix#L66

0 files changed, 0 insertions, 0 deletions