docs(airbyte-cdk): update migration guide (#45153)

Signed-off-by: Artem Inzhyyants <[email protected]>

Author: artem1205

airbyte-cdk/python/cdk-migrations.md

@@ -9,11 +9,23 @@ The changes to Airbyte CDK itself are backwards-compatible, but some changes are
 - uses the `airbyte_protocol` models directly, or `airbyte_cdk.models`, which points to `airbyte_protocol` models
 - uses third-party libraries, such as `pandas`, to read data from sources, which output non-native Python objects that cannot be serialized by the [orjson](https://github.com/ijl/orjson) library.
 
+> [!NOTE]
+> All Serializers have omit_none=True parameter that is applied recursively. Thus, all None values are excluded from output. 
+> This is expected behaviour and does not break anything in protocol.
+
 ### Updating direct usage of Pydantic based Airbyte Protocol Models
 
 If the connector uses Pydantic based Airbyte Protocol Models, the code will need to be updated to reflect the changes `pydantic`.
 It is recommended to import protocol classes not directly by `import airbyte_protocol` statement, but from `airbyte_cdk.models` package.
-It is also recommended to use `Serializers` from `airbyte_cdk.models` to manipulate the data or convert to/from JSON.
+It is also recommended to use *-`Serializer` from `airbyte_cdk.models` to manipulate the data or convert to/from JSON, e.g.
+```python3
+# Before (pydantic model message serialization) 
+AirbyteMessage().model_dump_json()
+
+# After (dataclass model serialization)
+orjson.dumps(AirbyteMessageSerializer.dump(AirbyteMessage())).decode()
+
+```
 
 ### Updating third-party libraries