update release note

Author: rachel-mack

source/whats-new.txt

@@ -29,9 +29,9 @@ What's New in 5.4
 The 5.4 driver release includes the following changes, fixes,
 and features:
 
-- Adds support for converting field names to snake case during serialization. To
-  learn more, see the :ref:`kotlin-sync-serialization-snake-case-eg` on the
-  Serialization page.
+- Adds ``BsonConfiguration`` support for bson-kotlinx snake case conversion
+  during serialization. To learn more, see the
+  :ref:`kotlin-sync-serialization-snake-case-eg` on the Serialization page.
 
 .. sharedinclude:: dbx/jvm/v5.4-wn-items.rst