The multi-user installation on macOS, which is now the only option, has
gotten complicated enough that it discourages some users from checking
Nix out for fear of being left with a "dirty" system. Detailed
uninstallation instructions should make this less of an issue.

Clarify that `/nix` being present after the uninstall is normal and it
will only disappear after a reboot.

Co-authored-by: Travis A. Everett <[email protected]>

The uninstall instructions used to accidentally remove the nix-darwin
LaunchDaemon, this was dropped. However, the original intent was to
remove the Store volume mounting LaunchDaemon.