Minor improve od documentation for network simulation.

Bug: webrtc:9630
Change-Id: I03827b890ab73662117864c16c59f15a9ae3aac8
Reviewed-on: https://webrtc-review.googlesource.com/96200
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24453}

api/test/simulated_network.h

@@ -73,6 +73,8 @@ class NetworkSimulationInterface {
   // Retrieves all packets that should be delivered by the given receive time.
   virtual std::vector<PacketDeliveryInfo> DequeueDeliverablePackets(
       int64_t receive_time_us) = 0;
+  // Returns time in microseconds when caller should call
+  // DequeueDeliverablePackets to get next set of packets to deliver.
   virtual absl::optional<int64_t> NextDeliveryTimeUs() const = 0;
   virtual ~NetworkSimulationInterface() = default;
 };