nixos/release-notes: explain the new system.nix file

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

@@ -4,6 +4,32 @@
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
+- The system.nix file has been added has an alternative entry point to configuration.nix (and flake.nix) that allows to configure 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`.
+  For example,
+  ```nix
+  # system.nix
+  let
+    # Pinned Nixpkgs archive
+    #
+    # Use `curl -I https://channels.nixos.org/nixos-26.05` to get the
+    # latest commit of the stable channel and `nix-prefetch-url --unpack`
+    # to compute its sha256 hash.
+    nixpkgs = builtins.fetchTarball {
+      url = "https://github.com/NixOS/nixpkgs/archive/c217913993d6.tar.gz";
+      sha256 = "026mprs324330pfazlgbw987qmsa8ligglarvqbcxzig2kgw0lqg";
+    };
+  in
+  import "${nixpkgs}/nixos" {
+    # Build NixOS using an external configuration.nix file,
+    # or directly set your options here
+    configuration = ./configuration.nix;
+  }
+  ```
+
+  The default location of system.nix is `/etc/nixos/system.nix` and can be changed by setting the `<nixos-system>` search path.
+  `nixos-rebuild` and `nixos-install` can now also load a system.nix file in the current directory (only if `--attr` is used) or from a directory specified with `--file`.
+
 - The default kernel package has been updated from 6.12 to 6.18. All supported kernels remain available.
 
 ## New Modules {#sec-release-26.05-new-modules}