Improve docs for client.Object

It was brought to my attention that the relationship between
client.Object and runtime.Object isn't particularly clear to folks who
are not used to working in the guts of client-go & apimachinery, so this
adds some docs to helpfully clarify and provide examples.

Author: DirectXMan12

pkg/client/object.go

@@ -21,15 +21,56 @@ import (
 	"k8s.io/apimachinery/pkg/runtime"
 )
 
-// Object is a Kubernetes object, allows functions to work indistinctly with any resource that
-// implements both Object interfaces.
+// Object is a Kubernetes object, allows functions to work indistinctly with
+// any resource that implements both Object interfaces.
+//
+// Semantically, these are objects which are both serializable (runtime.Object)
+// and identifiable (metav1.Object) -- think any object which you could write
+// as YAML or JSON, and then `kubectl create`.
+//
+// Code-wise, this means that any object which embeds both ObjectMeta (which
+// provides metav1.Object) and TypeMeta (which provides half of runtime.Object)
+// and has a `DeepCopyObject` implementation (the other half of runtime.Object)
+// will implement this by default.
+//
+// For example, nearly all the built-in types are Objects, as well as all
+// KubeBuilder-generated CRDs (unless you do something real funky to them).
+//
+// By and large, most things that implement runtime.Object also implement
+// Object -- it's very rare to have *just* a runtime.Object implementation (the
+// cases tend to be funky built-in types like Webhook payloads that don't have
+// a `metadata` field).
+//
+// Notice that XYZList types are distinct: they implement ObjectList instead.
 type Object interface {
 	metav1.Object
 	runtime.Object
 }
 
-// ObjectList is a Kubernetes object list, allows functions to work indistinctly with any resource that
-// implements both runtime.Object and metav1.ListInterface interfaces.
+// ObjectList is a Kubernetes object list, allows functions to work
+// indistinctly with any resource that implements both runtime.Object and
+// metav1.ListInterface interfaces.
+//
+// Semantically, this is any object which may be serialized (ObjectMeta), and
+// is a kubernetes list wrapper (has items, pagination fields, etc) -- think
+// the wrapper used in a response from a `kubectl list --output yaml` call.
+//
+// Code-wise, this means that any object which embedds both ListMeta (which
+// provides metav1.ListInterface) and TypeMeta (which provides half of
+// runtime.Object) and has a `DeepCopyObject` implementation (the other half of
+// runtime.Object) will implement this by default.
+//
+// For example, nearly all the built-in XYZList types are ObjectLists, as well
+// as the XYZList types for all KubeBuilder-generated CRDs (unless you do
+// something real funky to them).
+//
+// By and large, most things that are XYZList and implement runtime.Object also
+// implement ObjectList -- it's very rare to have *just* a runtime.Object
+// implementation (the cases tend to be funky built-in types like Webhook
+// payloads that don't have a `metadata` field).
+//
+// This is similar to Object, which is almost always implemented by the items
+// in the list themselves.
 type ObjectList interface {
 	metav1.ListInterface
 	runtime.Object