Documented mutation restriction for informer clients

Also brushed up some other informer comments for readability and
nitpicking accuracy.

Kubernetes-commit: c4774de94a80bb7d0f54a8a728ad3e97ad1a07b9

tools/cache/controller.go

@@ -183,9 +183,11 @@ func (c *controller) processLoop() {
 	}
 }
 
-// ResourceEventHandler can handle notifications for events that happen to a
-// resource. The events are informational only, so you can't return an
-// error.
+// ResourceEventHandler can handle notifications for events that
+// happen to a resource. The events are informational only, so you
+// can't return an error.  The handlers MUST NOT modify the objects
+// received; this concerns not only the top level of structure but all
+// the data structures reachable from it.
 //  * OnAdd is called when an object is added.
 //  * OnUpdate is called when an object is modified. Note that oldObj is the
 //      last known state of the object-- it is possible that several changes
@@ -205,7 +207,8 @@ type ResourceEventHandler interface {
 
 // ResourceEventHandlerFuncs is an adaptor to let you easily specify as many or
 // as few of the notification functions as you want while still implementing
-// ResourceEventHandler.
+// ResourceEventHandler.  This adapter does not remove the prohibition against
+// modifying the objects.
 type ResourceEventHandlerFuncs struct {
 	AddFunc    func(obj interface{})
 	UpdateFunc func(oldObj, newObj interface{})
@@ -237,6 +240,7 @@ func (r ResourceEventHandlerFuncs) OnDelete(obj interface{}) {
 // in, ensuring the appropriate nested handler method is invoked. An object
 // that starts passing the filter after an update is considered an add, and an
 // object that stops passing the filter after an update is considered a delete.
+// Like the handlers, the filter MUST NOT modify the objects it is given.
 type FilteringResourceEventHandler struct {
 	FilterFunc func(obj interface{}) bool
 	Handler    ResourceEventHandler