add warning on ObjectReference

Author: deads2k

staging/src/k8s.io/api/core/v1/generated.proto

@@ -4981,6 +4981,20 @@ type ServiceProxyOptions struct {
 }
 
 // ObjectReference contains enough information to let you inspect or modify the referred object.
+// ---
+// New uses of this type are discouraged because of difficulty describing its usage when embedded in APIs.
+//  1. Ignored fields.  It includes many fields which are not generally honored.  For instance, ResourceVersion and FieldPath are both very rarely valid in actual usage.
+//  2. Invalid usage help.  It is impossible to add specific help for individual usage.  In most embedded usages, there are particular
+//     restrictions like, "must refer only to types A and B" or "UID not honored" or "name must be restricted".
+//     Those cannot be well described when embedded.
+//  3. Inconsistent validation.  Because the usages are different, the validation rules are different by usage, which makes it hard for users to predict what will happen.
+//  4. The fields are both imprecise and overly precise.  Kind is not a precise mapping to a URL. This can produce ambiguity
+//     during interpretation and require a REST mapping.  In most cases, the dependency is on the group,resource tuple
+//     and the version of the actual struct is irrelevant.
+//  5. We cannot easily change it.  Because this type is embedded in many locations, updates to this type
+//     will affect numerous schemas.  Don't make new APIs embed an underspecified API type they do not control.
+// Instead of using this type, create a locally provided and used type that is well-focused on your reference.
+// For example, ServiceReferences for admission registration: https://github.com/kubernetes/api/blob/release-1.17/admissionregistration/v1/types.go#L533 .
 // +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
 type ObjectReference struct {
 	// Kind of the referent.