Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Clarify DefaultAddCapabilities behavior

Clarify that capabilities listed in `DefaultAddCapabilities` in the `PodSecurityPolicySpec` need not be repeated in `AllowedCapabilities`.

**Release note**:

```release-note
NONE
```