There actually is, just in a different spot.
You can find it in the documentation under Setup and configuration -> Dynatrace Managed -> Cluster API
Of course you can also use the integrated API Browser, if you open the context menu from the Cluster Management Console.
Doesn't seems to be as complete as Environment API Doc and Configuration API Doc though?
In CLuster API there are API to create user group and user, but these doesn't appear in the documentation
You are right, there are a few endpoints missing.
I'd recommend using the feedback tool embedded in the docs, that way it gets directly to the documentation team, and they are quite fast to react most of the time:
In the meantime, most of the information can also be found in the API Browser:
Hello! Cluster API doc isn't complete indeed. We're adding new endpoints gradually (user management endpoints are coming soon for example) and the public doc is generated based on the same spec as the API browser. However, we're working on the brand new Dynatrace documentation stack where we're adding full API docs automation with much better user experience and full coverage. This applies to all Dynatrace APIs. Can't give you a definite rollout time now, but it's coming.