[Initiative] Find a way to document values for serverless settings (that aren't exposed to users)

_This issue is part of a series of issue to improve Kibana settings docs. Refer to the parent for more context._

In serverless, kibana settings aren't exposed to users (besides advanced settings, which we have updated recently for serverless too).

That said, these settings are used under the hood, and sometimes they have a different default value/behavior than in other versions. A good example of that is https://github.com/elastic/docs-content-internal/issues/694.

We need to find a consistent yet clear way for users to understand those differences in behavior. Several options are possible, and we'd need to consider which one would be the clearest to users. Some of them might be familiar with settings if they know about self-managed or ECH, others might be all new to Elastic and would simply not know how to check/where to look for these default behaviors.

- Broaden the self-managed/ECH settings page to serverless and indicate clearly when something has a different value in serverless?
- Have specific sections in the various app docs that summarizes these values (perhaps are limitations or simple information)? If users can't edit them nor map the behavior to a setting, this might be a better approach to make the information findable.
- Else?