doc: Add detailed uninstall section for macOS

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.

doc/manual/src/installation/installing-binary.md

@@ -84,7 +84,9 @@ The installer will modify `/etc/bashrc`, and `/etc/zshrc` if they exist.
 The installer will first back up these files with a `.backup-before-nix`
 extension. The installer will also create `/etc/profile.d/nix.sh`.
 
-You can uninstall Nix with the following commands:
+## Uninstalling
+
+### Linux
 
 ```console
 sudo rm -rf /etc/profile/nix.sh /etc/nix /nix ~root/.nix-profile ~root/.nix-defexpr ~root/.nix-channels ~/.nix-profile ~/.nix-defexpr ~/.nix-channels
@@ -95,15 +97,87 @@ sudo systemctl stop nix-daemon.service
 sudo systemctl disable nix-daemon.socket
 sudo systemctl disable nix-daemon.service
 sudo systemctl daemon-reload
-
-# If you are on macOS, you will need to run:
-sudo launchctl unload /Library/LaunchDaemons/org.nixos.nix-daemon.plist
-sudo rm /Library/LaunchDaemons/org.nixos.nix-daemon.plist
 ```
 
 There may also be references to Nix in `/etc/profile`, `/etc/bashrc`,
 and `/etc/zshrc` which you may remove.
 
+### macOS
+
+1. Edit `/etc/zshrc` and `/etc/bashrc` to remove the lines sourcing
+   `nix-daemon.sh`, which should look like this:
+
+   ```bash
+   # Nix
+   if [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ]; then
+     . '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'
+   fi
+   # End Nix
+   ```
+
+   If these files haven't been altered since installing Nix you can simply put
+   the backups back in place:
+
+   ```console
+   sudo mv /etc/zshrc.backup-before-nix /etc/zshrc
+   sudo mv /etc/bashrc.backup-before-nix /etc/bashrc
+   ```
+
+   This will stop shells from sourcing the file and bringing everything you
+   installed using Nix in scope.
+
+2. Stop and remove the Nix daemon services:
+
+   ```console
+   sudo launchctl unload /Library/LaunchDaemons/org.nixos.nix-daemon.plist
+   sudo rm /Library/LaunchDaemons/org.nixos.nix-daemon.plist
+   sudo launchctl unload /Library/LaunchDaemons/org.nixos.activate-system.plist
+   sudo rm /Library/LaunchDaemons/org.nixos.activate-system.plist
+   ```
+
+   This stops the Nix daemon and prevents it from being started next time you
+   boot the system.
+
+3. Remove the `nixbld` group and the `_nixbuildN` users:
+
+   ```console
+   sudo dscl . -delete /Groups/nixbld
+   for u in $(sudo dscl . -list /Users | grep _nixbld); do sudo dscl . -delete /Users/$u; done
+   ```
+
+   This will remove all the build users that no longer serve a purpose.
+
+4. Edit fstab using `sudo vifs` to remove the line mounting the Nix Store
+   volume on `/nix`, which looks like this,
+   `LABEL=Nix\040Store /nix apfs rw,nobrowse`. This will prevent automatic
+   mounting of the Nix Store volume.
+
+5. Edit `/etc/synthetic.conf` to remove the `nix` line. If this is the only
+   line in the file you can remove it entirely, `sudo rm /etc/synthetic.conf`.
+   This will prevent the creation of the empty `/nix` directory to provide a
+   mountpoint for the Nix Store volume.
+
+6. Remove the files Nix added to your system:
+
+   ```console
+   sudo rm -rf /etc/nix /var/root/.nix-profile /var/root/.nix-defexpr /var/root/.nix-channels ~/.nix-profile ~/.nix-defexpr ~/.nix-channels
+   ```
+
+   This gets rid of any data Nix may have created except for the store which is
+   removed next.
+
+7. Remove the Nix Store volume:
+   
+   ```console
+   sudo diskutil apfs deleteVolume /nix
+   ```
+
+   This will remove the Nix Store volume and everything that was added to the
+   store.
+
+The change to `/etc/synthetic.conf` will only take effect after a reboot but
+you shouldn't have any traces of Nix left on your system.
+
 # macOS Installation <a name="sect-macos-installation-change-store-prefix"></a><a name="sect-macos-installation-encrypted-volume"></a><a name="sect-macos-installation-symlink"></a><a name="sect-macos-installation-recommended-notes"></a>
 <!-- Note: anchors above to catch permalinks to old explanations -->