Add note about pod annotations for more clarity
What does this MR do?
Adds a note about pod_annotations_overwrite_allowed to the section https://docs.gitlab.com/runner/executors/kubernetes.html#overwriting-pod-annotations
Why was this MR needed?
It is not clear that user should set pod_annotations_overwrite_allowed
to be able to use pod annotations override in .gitlab-ci.yml
.