[Documentation] Document where the deserialization is not emitted

[dej611]

Question

In the current observability documentation at https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current/observability.html within the deserialization event the following quote is present:

(This event might not be emitted in certain situations).

It would be useful to know when such event is not emitted. Use case: we're trying to instrument Kibana to know how much time is spent on the server side (included the detail about deserialization of the response) and it needs to know when to fallback.