Update CHANGELOG.md

Author: alisdair

CHANGELOG.md

@@ -9,6 +9,7 @@ ENHANCEMENTS:
 
 * The "Invalid for_each argument" error message for unknown maps/sets now includes an additional paragraph to try to help the user notice they can move apply-time values into the map _values_ instead of the map _keys_, and thus avoid the problem without resorting to `-target`. [GH-30327]
 * When showing the progress of a remote operation running in Terraform Cloud, Terraform CLI will include information about post-plan [run tasks](https://www.terraform.io/cloud-docs/workspaces/settings/run-tasks). [GH-30141]
+* Error messages for preconditions, postconditions, and custom variable validations are now evaluated as expressions, allowing interpolation of relevant values into the output. [GH-30613]
 
 BUG FIXES: