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>.

kubadm - add comment for etcd server cert clientauth usage workaround

Adds TODO requested in https://github.com/kubernetes/kubernetes/pull/65020#discussion_r194801712

**Release note**:
```release-note
NONE
```