Add missing comments in APF API types

pkg/apis/flowcontrol/types.go

@@ -171,13 +171,17 @@ type PolicyRulesWithSubjects struct {
 // ways of matching an originator; by user, group, or service account.
 // +union
 type Subject struct {
+	// `kind` indicates which one of the other fields is non-empty.
 	// Required
 	// +unionDiscriminator
 	Kind SubjectKind
+	// `user` matches based on username.
 	// +optional
 	User *UserSubject
+	// `group` matches based on user group name.
 	// +optional
 	Group *GroupSubject
+	// `serviceAccount` matches ServiceAccounts.
 	// +optional
 	ServiceAccount *ServiceAccountSubject
 }