Add warning to Uninstall instructions about macOS behavior

[lilyball]

The install instructions today simply say to rm -rf /nix.

nixos-homepage/download.tt

Lines 49 to 55 in ef73dd4

	<article id="nix-uninstall">
	<h2>Uninstall</h2>
	<p>
	You can uninstall Nix simply by running <code>rm -rf /nix</code>.
	</p>
	<!-- TODO: expand this https://github.com/NixOS/nix/issues/1402 -->
	</article>

This does not work on macOS, where /nix is a mounted volume. The issue referenced in the HTML comment (NixOS/nix#1402) has some macOS-specific information, though really the canonical uninstall instructions come from the create-darwin-volume.sh script used during install:

        echo "  1. Remove the entry from fstab using 'sudo vifs'"
        echo "  2. Destroy the data volume using 'diskutil apfs deleteVolume'"
        echo "  3. Remove the 'nix' line from /etc/synthetic.conf or the file"

(note that these steps will change with NixOS/nix#4181).

Ideally we'd keep the website updated with the full set of steps, but in the meantime, we should at least add a caveat here saying that these instructions aren't sufficient on macOS and point the user toward somewhere else (such as NixOS/nix#1402). Once NixOS/nix#4181 is merged our installer approach for macOS should be stabilized and we can then update the website with more explicit instructions.

[garbas]

@lilyball 👍 Would you be so kind and try to provide a PR? I'm currently a bit overwhelmed with the work and any help is much appreciated.

[edolstra]

If we need extensive uninstall instructions, it's probably better to add them to the Nix manual (and link to them from here), to avoid ending up with two diverging sources of truth.