DOCS-15368 - Clarify UUID changes (#402)

(cherry picked from commit b30dc6e)

Author: mongoKart

source/upgrade.txt

@@ -89,6 +89,16 @@ Version 4.0 Breaking Changes
   UUID values must explicitly specify which representation to use. You can
   specify the representation in the ``uuidRepresentation`` property of
   ``MongoClientSettings``.
+
+  The UUID representation that you specify strictly controls how the driver
+  decodes UUIDs. In previous versions of the driver, if you specified the
+  ``JAVA_LEGACY`` representation, the driver would decode binary objects
+  of subtypes 3 and 4 as UUIDs. In version 4.0, the ``JAVA_LEGACY``
+  representation works only with subtype 3.
+  
+  For a list of members in the ``UuidRepresentation`` enum, see the
+  `API documentation. <https://http://mongodb.github.io/mongo-java-driver/4.0/apidocs/bson/org/bson/UuidRepresentation.html>`__
+
 - The connection pool no longer restricts the number of wait queue threads
   or asynchronous tasks that require a connection to MongoDB. The
   application should throttle requests as necessary rather than depend on