Added notice about token in kubernetes.md

What does this MR do?

Clarifies documentations on how to setup a runner on kubernetes

Why was this MR needed?

To avoid confusion made in myself and described in gitlab-org/gitlab-ce#37807 and some other issues

Are there points in the code the reviewer needs to double check?

No

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#37807