Note on Manager#GetClient about delegating clients

While the concrete Options struct documents delegating behavior, and the
main overview documents it as well, this adds a note to manager#GetClient
that it may return a non-live client, just to avoid confusion.

pkg/manager/manager.go

@@ -63,7 +63,10 @@ type Manager interface {
 	// GetScheme returns an initialized Scheme
 	GetScheme() *runtime.Scheme
 
-	// GetClient returns a client configured with the Config
+	// GetClient returns a client configured with the Config. This client may
+	// not be a fully "direct" client -- it may read from a cache, for
+	// instance.  See Options.NewClient for more information on how the default
+	// implementation works.
 	GetClient() client.Client
 
 	// GetFieldIndexer returns a client.FieldIndexer configured with the client