[user-authn] Remove misleading info from docs

[unreturned]

Description

Remove misleading information from docs. Created user does not have such permissions:

kubectl --kubeconfig /tmp/myconf get namespaces
Error from server (Forbidden): namespaces is forbidden: User "[email protected]" cannot list resource "namespaces" in API group "" at the cluster scope

Why do we need it, and what problem does it solve?

Improve documentation

Why do we need it in the patch release (if we do)?

Not necessarily

What is the expected result?

Checklist

• [ ] The code is covered by unit tests.
• [ ] e2e tests passed.
• [ ] Documentation updated according to the changes.
• [ ] Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: user-authn
type: chore
summary: Remove misleading info from docs.
impact_level: low