uninstall documentation is inaccurate on macOS multi user install

[ghost]

The manual states

You can uninstall Nix simply by running rm -rf /nix

On macOS after a multi user install we found that this was not the case. The install script prompted us to remove other configuration directories, and restore backed up system bash and zsh profiles. Additionally, we needed to login to a new terminal to unset existing NIX_ environment variables.
I propose we add the needed explanation to that chapter from the current install script.

update

Possibly duplicate of #1551

[pjgoodall]

1. If a script installed nix, then there should be a script to uninstall it. Otherwise you've pushed beginners down a cliff.
2. The existence of an effective uninstall-script which reverses the install-script should be a requirement for a platform release.
3. Of-course the result of install, and an uninstall procedure should be documented.

[stale]

I marked this as stale due to inactivity. → More info

[nixos-discourse]

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

https://discourse.nixos.org/t/anyone-up-for-picking-at-some-nix-onboarding-improvements/13152/6