EDU-3747: [ Documentation request ] Update Message Handler coverage to clarify use of data class parameters

[antmendoza]

Brief description

https://docs.temporal.io/develop/python/message-passing#writing-message-handlers

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.

While this is true, it can get the reader the impression that adding fields to dataclasses will always work. However, in reality, they could encounter serialization/deserialization issues.

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We should add that serialization/deserialization can fail with the default data converter if the new field does not have a default value.

[fairlydurable]

Great suggestion. Thanks for submitting it. Tracking as EDU-3747.