DOCSP-46065: GUID serialization breaking change note (#439)

Author: mcmorisi

source/upgrade/v3.txt

@@ -248,3 +248,13 @@ Version 3.0 Breaking Changes
   - The driver seals the ``MongoClient``, ``MongoDatabase``, and ``MongoCollection`` classes.
     We recommend using the ``IMongoClient``, ``IMongoDatabase``, and ``IMongoCollection``
     interfaces directly.
+
+- The driver requires applications to explicitly configure how to serialize GUIDs
+  by using the ``GuidSerializer`` and ``ObjectSerializer`` classes. {+driver-short+}
+  users building new applications can implement a global ``GuidSerializer`` by registering
+  it. We recommend that users with older applications verify that all ``GUIDs`` are
+  serialized the same way when implementing a global ``GuidSerializer``. You can also
+  configure ``GUID`` serialization on a case-by-case basis without registering a global
+  ``GuidSerializer``.
+  
+  To learn more about GUID serialization, see the :ref:`csharp-guids` guide.