[Docs] Improve documentation about Serializer

[stephanvierkant]

In my Symfony project, my entity model contains attributes for Doctrine ORM, API Platform, Validator Constraints, and more. Therefor I used the Serializer to populate the indexes of my OpenSearch cluster. This works great, but it has some drawbacks. I found the documentation not very clear about this.

I'd like to contribute to the documentation by adding more information about how to use the Serializer, but I'd like to get some feedback from the community. Are you using the serializer (Symfony or JMS) or not? What's your experience? Are you using TransformEvent?