diff --git a/doc/manual/src/release-notes/rl-2.12.md b/doc/manual/src/release-notes/rl-2.12.md
index 57d092e01..e1e3efe1a 100644
--- a/doc/manual/src/release-notes/rl-2.12.md
+++ b/doc/manual/src/release-notes/rl-2.12.md
@@ -2,8 +2,19 @@
 
 * On Linux, Nix can now run builds in a user namespace where they run
   as root (UID 0) and have 65,536 UIDs available.
+  This is primarily useful for running containers such as `systemd-nspawn`
+  inside a Nix build. For an example, see [`tests/systemd-nspawn/nix`][nspawn].
 
-  This can be used by requiring `uid-range` [system feature] in derivations.
+  [nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.
+
+  A build can enable this by setting the derivation attribute:
+
+  ```
+  requiredSystemFeatures = [ "uid-range" ];
+  ```
+
+  The `uid-range` [system feature] requires the [`auto-allocate-uids`]
+  setting to be enabled.
 
   [system feature]: ../command-ref/conf-file.md#conf-system-features