Merge pull request #58103 from php-coder/describe_group_and_version_fields

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

cmd/kube-apiserver/app/aggregator.go

@@ -187,8 +187,12 @@ func makeAPIServiceAvailableHealthzCheck(name string, apiServices []*apiregistra
 	})
 }
 
+// priority defines group priority that is used in discovery. This controls
+// group position in the kubectl output.
 type priority struct {
-	group   int32
+	// group indicates the order of the group relative to other groups.
+	group int32
+	// version indicates the relative order of the version inside of its group.
 	version int32
 }
 
@@ -229,6 +233,9 @@ var apiVersionPriorities = map[schema.GroupVersion]priority{
 	{Group: "admissionregistration.k8s.io", Version: "v1beta1"}:  {group: 16700, version: 12},
 	{Group: "admissionregistration.k8s.io", Version: "v1alpha1"}: {group: 16700, version: 9},
 	{Group: "scheduling.k8s.io", Version: "v1alpha1"}:            {group: 16600, version: 9},
+	// Append a new group to the end of the list if unsure.
+	// You can use min(existing group)-100 as the initial value for a group.
+	// Version can be set to 9 (to have space around) for a new group.
 }
 
 func apiServicesToRegister(delegateAPIServer genericapiserver.DelegationTarget, registration autoregister.AutoAPIServiceRegistration) []*apiregistration.APIService {