Fix comments

pubsublite/ps/settings.go

@@ -119,7 +119,8 @@ func (s *PublishSettings) toWireSettings() wire.PublishSettings {
 // NackHandler is invoked when pubsub.Message.Nack() is called. Cloud Pub/Sub
 // Lite does not have a concept of 'nack'. If the nack handler implementation
 // returns nil, the message is acknowledged. If an error is returned, the
-// SubscriberClient will will consider this a fatal error and terminate.
+// SubscriberClient will terminate once all outstanding message receivers have
+// finished and outstanding messages have been ack or nacked.
 //
 // In Cloud Pub/Sub Lite, only a single subscriber for a given subscription is
 // connected to any partition at a time, and there is no other client that may
@@ -160,8 +161,7 @@ type ReceiveSettings struct {
 	Partitions []int
 
 	// Optional custom function to handle pubsub.Message.Nack() calls. If not set,
-	// the default behavior is to immediately terminate the SubscriberClient with
-	// a fatal error.
+	// the default behavior is to terminate the SubscriberClient.
 	NackHandler NackHandler
 
 	// Optional custom function that transforms a PubSubMessage API proto to a