[doc] update migration doc

Python SDK architect provide some suggestions in e-mail `RE: Confirmation about Azure MGMT Python SDK release for brownfield service which already converts from Swagger to Typespec` about migration doc for Typespec:
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We can improve on the descriptions as to why we are making the breaking changes; most of them are (ironically) to avoid future breaking changes; e.g.

* additional_properties captured properties not yet documented. Unfortunately, that had the effect of breaking callers that fished them out from additional_properties when we added documentation for the property.
* flattening had similar problems when new properties were added to what used to be a flattened model.
* the serialize/deserialize ended were not entirely symmetrical leading to hard to debug failures if callers tried to use them to store raw data.

I would not claim that keyword-only arguments give IDEs better intellisense; the reason we make them keyword-only was also because it helped the models evolve over time. Previously, making a parameter optional could cause a breaking change.

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