Automatic merge from submit-queue (batch tested with PRs 56400, 56535). 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>.

Document None dns policy clearly 

**What this PR does / why we need it**:

Needs to indicate that None dns policy is controlled by a feature gate and by alpha. It also needs to note that only 1.9 or later Kubernetes servers will allow the None value.

Needs to be fixed for 1.9 so that public API documentation is correct.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #56399

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```