users guide: Note improved constant folding in 8.6 release notes

docs/users_guide/8.6.1-notes.rst

@@ -79,6 +79,10 @@ Compiler
 
 - GHC now supports British spelling of :extension:`GeneralizedNewtypeDeriving`.
 
+- GHC now does significantly more constant folding in its core-to-core optimiser.
+  This will result in significantly better code being generated for some
+  programs. See :ghc-ticket:`9136`.
+
 - The code-generation effects of :ghc-flag:`-dynamic` can now be
   enabled independently by the flag
   :ghc-flag:`-fexternal-dynamic-refs`. If you don't know why you might