doc: add disabling of emacs' GC mark trace buffer to 25.11 release notes

doc/release-notes/rl-2511.section.md

@@ -265,6 +265,8 @@
   recommendation](https://clickhouse.com/docs/faq/operations/production). Users
   can continue to use the `clickhouse-lts` package if desired.
 
+- `emacs` now disables the GC mark trace buffer by default. This improves GC performance by 5%, but can make GC issues harder to debug. This is configurable with `withGcMarkTrace`.
+
 - `buildPythonPackage` and `buildPythonApplication` now default to `nix-update-script` as their default `updateScript`. This should improve automated updates, since nix-update is better maintained than the in-tree update script and has more robust fetcher support.
 
 ## Nixpkgs Library {#sec-nixpkgs-release-25.11-lib}