update upgrade guide

Author: gstvg

docs/source/library-user-guide/upgrading/54.0.0.md

@@ -787,3 +787,26 @@ a combined `OR` (e.g., index-backed sources).
 ```sql
 SET datafusion.optimizer.enable_unions_to_filter = true;
 ```
+
+### The `RegisterFunction` enum has a new `HigherOrder` variant
+
+`RegisterFunction` now has a `HigherOrder(Arc<dyn HigherOrderUDF>)` variant
+so user-defined higher-order functions can be registered
+
+**Who is affected:**
+
+- Users who match on a `RegisterFunction` without a default branch `_ => {}`
+
+**Migration guide:**
+
+Add the new branch to the match with the logic applicable to the context.
+
+```diff
+match register_function {
+    RegisterFunction::Scalar(scalar) => {},
+    RegisterFunction::Aggregate(aggregate) => {},
+    RegisterFunction::Window(window) => {},
++   RegisterFunction::HigherOrder(higher_order) => {},
+    RegisterFunction::Table(name, table) => {},
+}
+```