Update the service account issuer discovery documentation

[olemarkus]

Fixes #11623

[johngmyers]

Have we verified Istio sets the API audience correctly?

[olemarkus]

Only kOps documentation on istio says anything about setting audience directly on the api server. It would also be very hard to achieve this on say managed kubernetes.

[johngmyers]

Istio documentation also mentions setting the API audience. They seem to have copied from us.

[olemarkus]

That's the docs I referred to that was kops specific (and now incorrect). The linked issue also suggested that this worked without setting audience. Could ask there to confirm audience is really not set.

[johngmyers]

Saw the confirmation in the linked issue.

[k8s-ci-robot]

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[johngmyers]

Filed istio/istio.io#9859