docs: explain how to enable flakes on NixOS

Since `nix.conf` is generated when using NixOS, it cannot be edited.

Instead we add the instructions described by Xe in
https://xeiaso.net/blog/nix-flakes-1-2022-02-21

docs/nix-flakes.adoc

@@ -13,12 +13,24 @@ The support is still experimental and may change in backwards incompatible ways.
 
 * Enable experimental features `nix-command` and `flakes`.
 +
-Either set in `nix.conf`
+** When using NixOS, add the following to your `configuration.nix` and rebuild your system.
++
+[source,nix]
+nix = {
+  package = pkgs.nixFlakes;
+  extraOptions = ''
+    experimental-features = nix-command flakes
+  '';
+};
++
+** If you are not using NixOS, add the following to `nix.conf` (located at `~/.config/nix/` or `/etc/nix/nix.conf`).
 +
 [source,bash]
 experimental-features = nix-command flakes
 +
-or pass them to `nix` and `home-manager` by
+You may need to restart the Nix daemon with, for example, `sudo systemctl restart nix-daemon.service`.
++
+** Alternatively, you can enable flakes on a per-command basis with the following additional flags to `nix` and `home-manager`:
 +
 [source,console]
 ----