doc/rl-2605: document /dev/root unavailability with systemd stage 1

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I25f6196bc5af3a6cadff1972c1ab1f9b6a6a6964

nixos/doc/manual/release-notes/rl-2605.section.md

@@ -13,6 +13,7 @@
   - If you use LUKS disk encryption, ensure that `fileSystems."/".device` is set to `"/dev/mapper/<name>"`, where `<name>` matches the name in your `boot.initrd.luks.devices.<name>` definition, to avoid systemd timing out while prompting for a passphrase. If you have a more complex setup, e.g. with LVM on top of LUKS, you may need to add `"x-systemd.device-timeout=infinity"` to `fileSystems."/".options` instead. If you need to disable the timeout before you can boot into the system, pass `systemd.default_device_timeout_sec=infinity` on the kernel command line.
   - The `cryptsetup-askpass` program is not available; use `systemctl default` instead, which will prompt for passphrases as necessary. If you pipe password responses into SSH over stdin, use `ssh -o RequestTTY=force` to ensure `systemctl default` gets a TTY to prompt on.
   - Many kernel parameters have been replaced with native systemd versions; see [](#sec-boot-problems).
+  - `/dev/root` is not available with the systemd stage 1. In the old scripted stage 1, `/dev/root` was a symlink created by the init script from the `root=` kernel command line. With systemd stage 1, this symlink is not provided. If your configuration uses `/dev/root` in `fileSystems`, replace it with a stable device path such as `/dev/disk/by-label/...`, `/dev/disk/by-uuid/...`, or the appropriate `/dev/mapper/...` path.
 
 - The system.nix file has been added as an alternative entry point to configuration.nix (and flake.nix) that allows configuring NixOS without using `nix-channel`.
   This file must evaluate to a NixOS system derivation or an attribute set of such derivations, in which case the attribute to build has to be selected with the `--attr` option of `nixos-rebuild` or `nixos-install`.