Add explain for preempt sunction.

plugin/pkg/scheduler/scheduler.go

@@ -184,6 +184,9 @@ func (sched *Scheduler) schedule(pod *v1.Pod) (string, error) {
 	return host, err
 }
 
+// preempt tries to create room for a pod that has failed to schedule, by preempting lower priority pods if possible.
+// If it succeeds, it adds the name of the node where preemption has happened to the pod annotations.
+// It returns the node name and an error if any.
 func (sched *Scheduler) preempt(preemptor *v1.Pod, scheduleErr error) (string, error) {
 	if !utilfeature.DefaultFeatureGate.Enabled(features.PodPriority) {
 		glog.V(3).Infof("Pod priority feature is not enabled. No preemption is performed.")