Add note about empty runner-registration-token

What does this MR do?

We have multiple users trying to solve a problem when their secret storage solution doesn't allow them to set empty string as a value for secret.

This isn't actually a problem, they just need to use new token architecture(we've already updated docs to it), and they can set anything for runner-registration-token, runner-token will take precedence anyway.

This MR clarifies the docs

What are the relevant issue numbers?

• Change order of scripts in entrypoint to allow ... (gitlab-org/charts/gitlab-runner#462 - closed)
• Add support for the use of secrets provisioned ... (gitlab-org/charts/gitlab-runner#456 - closed)

docs/install/kubernetes.md

@@ -552,6 +552,10 @@ runners:
 This example uses the secret `gitlab-runner-secret` and takes the value of
 `runner-token` to register the runner.
 
+NOTE:
+If your secret management solution doesn't allow you to set empty string for `runner-registration-token`,
+you can set it to any string - it will be ignored anyway.
+
 ### Switching to the Ubuntu-based `gitlab-runner` Docker image
 
 By default the GitLab Runner Helm Chart uses the Alpine version of the `gitlab/gitlab-runner` image,