perf(message passing): added detail around data classes and serialization (#4325)

Author: flippedcoder

docs/develop/dotnet/message-passing.mdx

@@ -37,8 +37,7 @@ Follow these guidelines when writing your message handlers:
 
 - Message handlers are defined as methods on the Workflow class, using one of the three attributes: [`WorkflowQueryAttribute`](https://dotnet.temporal.io/api/Temporalio.Workflows.WorkflowQueryAttribute.html), [`WorkflowSignalAttribute`](https://dotnet.temporal.io/api/Temporalio.Workflows.WorkflowSignalAttribute.html), and [`WorkflowUpdateAttribute`](https://dotnet.temporal.io/api/Temporalio.Workflows.WorkflowUpdateAttribute.html).
 - The parameters and return values of handlers and the main Workflow function must be [serializable](/dataconversion).
-- Prefer data classes to multiple input parameters.
-  Data class parameters allow you to add fields without changing the calling signature.
+- Prefer data classes to multiple input parameters. Data class parameters allow you to add fields without changing the calling signature. Keep in mind that serialization and deserialization can fail with the default data converter if the new field does not have a default value.
 
 ### Query handlers {#queries}
 

docs/develop/python/message-passing.mdx

@@ -42,8 +42,7 @@ Follow these guidelines when writing your message handlers:
 
 - Message handlers are defined as methods on the Workflow class, using one of the three decorators: [`@workflow.query`](https://python.temporal.io/temporalio.workflow.html#query), [`@workflow.signal`](https://python.temporal.io/temporalio.workflow.html#signal), and [`@workflow.update`](https://python.temporal.io/temporalio.workflow.html#update).
 - The parameters and return values of handlers and the main Workflow function must be [serializable](/dataconversion).
-- Prefer [data classes](https://docs.python.org/3/library/dataclasses.html) to multiple input parameters.
-  Data class parameters allow you to add fields without changing the calling signature.
+- Prefer [data classes](https://docs.python.org/3/library/dataclasses.html) to multiple input parameters. Data class parameters allow you to add fields without changing the calling signature. Keep in mind that serialization and deserialization can fail with the default data converter if the new field does not have a default value.
 
 ### Query handlers {#queries}