various: document /dev/root unavailability; provide migration steps (#526310)

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`.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -787,9 +787,10 @@ in
       options = [ "mode=0755" ];
     };
 
-    # Note that /dev/root is a symlink to the actual root device
-    # specified on the kernel command line, created in the stage 1
-    # init script.
+    # With systemd stage 1, the ISO is identified by its volume label.
+    # With the scripted stage 1, /dev/root is a symlink to the actual
+    # root device specified on the kernel command line, created by the
+    # stage 1 init script.
     "/iso" = lib.mkImageMediaOverride {
       device =
         if config.boot.initrd.systemd.enable then