doc/rl2511: update allowVariants entry (#409484)

Author: RossComputerGuy

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

@@ -3,7 +3,7 @@
 ## Highlights {#sec-nixpkgs-release-25.11-highlights}
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
-- Added `allowVariants` to gate availability of package sets like `pkgsLLVM`, `pkgsMusl`, `pkgsZig`, etc.
+- Added `allowVariants` to gate availability of package sets like `pkgsLLVM`, `pkgsMusl`, `pkgsZig`, etc. This option will be removed in a future release.
 
 ## Backward Incompatibilities {#sec-nixpkgs-release-25.11-incompatibilities}