chore: add comments to help explain durability guarantees of the cons…

…umer (#15105)

pkg/ingester/kafka_consumer.go

@@ -73,6 +73,10 @@ func (kc *kafkaConsumer) Start(ctx context.Context, recordsChan <-chan []partiti
 		for {
 			select {
 			case <-ctx.Done():
+				// It can happen that the context is canceled while there are unprocessed records
+				// in the channel. However, we do not need to process all remaining records,
+				// and can exit out instead, as partition offsets are not committed until
+				// the record has been handed over to the Pusher and committed in the WAL.
 				level.Info(kc.logger).Log("msg", "shutting down kafka consumer")
 				return
 			case records := <-recordsChan: