Update release notes wrt #3791

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -9,6 +9,8 @@ Not yet released
 #3566: Cannot use both `JsonCreator.Mode.DELEGATING` and `JsonCreator.Mode.PROPERTIES`
   static creator factory methods for Enums
  (reported by @andrewbents)
+#3791: Flush readonly map together with shared on `SerializerCache.flush()`
+ (suggested by @vdaniloff)
 #3814: Enhance `StdNodeBasedDeserializer` to support `readerForUpdating`
  (requested by Matt N)
  (contributed by Joo-Hyuk K)