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>.
Add comments for explaining the group/version fields
**What this PR does / why we need it**:
This PR adds comments for explaining group/version fields. This is my understanding of what @deads2k told me today. Please, correct me if I'm wrong or you see how it can be further improved.
I didn't find where it's documented, so I hope that this PR makes it easy for other new contributors addition of new APIs.
**Release note**:
```release-note
NONE
```
PTAL @deads2k
CC @simo5
e290741719