Minor cleanups.
Minor cleanups.

- ValidateNoNewFinalizers: remove unused const
- Mention that mutation of spec.initContainers[*].image field is allowed
- Improve godoc comments
- validation_test.go(expectPrefix): remove unused function

Kubernetes-commit: 95049b64408cd90f9a6c1d10b3b0bb35bba44ad5

kubernetes/apimachinery