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