Custom serialization docs needs deepening

[Danthar]

The custom serialization document currently only explains how you can configure a certain type to be serialized a custom serializer.

But it does not explain what the default serializer is (json.net). And why a production user would want to to pick a different serializer. So explanation about message versioning concerns, why certain serializers are better suited, etc.

/articles/persistence/custom-serialization.html