Qualify deployment-specific details in API descriptions

[lcawl]

Motivation

Since the same specifications are used to generate both the elasticsearch-openapi.json and elasticsearch-serverless-openapi.json, it's possible for some features that are not supported in Serverless, for example, to be mentioned in the descriptions.

There's information about the feature availability within Serverless here: https://www.elastic.co/guide/en/serverless/current/elasticsearch-differences.html#elasticsearch-differences-serverless-feature-categories

Ideally the way that deployment-specific details are described in API docs should align with how this is accomplished in the other docs per https://docs-v3-preview.elastic.dev/elastic/docs-builder/tree/main/versions/content-patterns#prose

Example

_search API discusses Cross Cluster Search, which is not supported for serverless.