Docs: Capture Gustavo's feedback for encoding/marshalling

https://github.com/infinispan/infinispan/pull/9106 - This PR was merged before Gustavo was able to complete his review.

There's also a mention of the `application/unknown` media type that we should remove and be more generic about - we agreed during review that this media type should not be leaked in the documentation.

https://github.com/infinispan/infinispan/pull/9106#discussion_r633514496

https://github.com/infinispan/infinispan/pull/9106#discussion_r633521518

https://github.com/infinispan/infinispan/pull/9106#discussion_r633549737