Update custom entrypoint documentation for Kubernetes executor
What does this MR do?
Update the documentation about the use of the image entrypoint when running a job with the k8s executor.
Why was this MR needed?
The information available in the documentation was outdated
What's the best way to test this MR?
What are the relevant issue numbers?
https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/27