doc/rl2505: move variants to 25.05

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

@@ -437,6 +437,10 @@
   There is also a breaking change in the handling of CUDA. Instead of using a CUDA compatible jaxlib
   as before, you can use plugins like `python3Packages.jax-cuda12-plugin`.
 
+- Added `allowVariants` to gate availability of package sets like `pkgsLLVM`, `pkgsMusl`, `pkgsZig`, etc. This was done in an effort to
+  decrease evaluation times by limiting the number of instances of nixpkgs to evaluate. The option will be removed in the future as a
+  new mechanism is in the works for handling cross compilation.
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
 ## Other Notable Changes {#sec-nixpkgs-release-25.05-notable-changes}