Comment default behaviour of GracePeriodSeconds

It took me a while to spot this subtlety.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

Kubernetes-commit: 7c764c8550121b48a6e2678700b6a80fa7d3e2f0

pkg/drain/drain.go

@@ -49,7 +49,12 @@ type Helper struct {
 	Ctx    context.Context
 	Client kubernetes.Interface
 	Force  bool
+
+	// GracePeriodSeconds is how long to wait for a pod to terminate.
+	// IMPORTANT: 0 means "delete immediately"; set to a negative value
+	// to use the pod's terminationGracePeriodSeconds.
 	GracePeriodSeconds int
+
 	IgnoreAllDaemonSets bool
 	Timeout             time.Duration
 	DeleteEmptyDirData  bool