doc/release-notes: mention nixpkgs.source

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -29,6 +29,8 @@
 - A `nixos-rebuild build-image` sub-command has been added.
   It allows users to build platform-specific (disk) images from their NixOS configurations. `nixos-rebuild build-image` works similar to the popular [nix-community/nixos-generators](https://github.com/nix-community/nixos-generators) project. See new [section on image building in the nixpkgs manual](https://nixos.org/manual/nixpkgs/unstable/#sec-image-nixos-rebuild-build-image). It is also available for `nixos-rebuild-ng`.
 
+- Using the new `nixpkgs.source` option it is finally possible to build NixOS with a specific Nixpkgs version declaratively, without depending on nix-channel or flakes.
+
 - `nixos-option` has been rewritten to a Nix expression called by a simple bash script. This lowers our maintenance threshold, makes eval errors less verbose, adds support for flake-based configurations, descending into `attrsOf` and `listOf` submodule options, and `--show-trace`.
 
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