Comment on PacketSocketFactory injection.

Document how PeerConnectionFactoryDependencies::packet_socket_factory
interacts with injected port allocator.

Bug: webrtc:7447
Change-Id: Id79b345c1b708944c84f466680c4b3fba77e4feb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/266480
Commit-Queue: Niels Moller <nisse@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37298}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index 8a6ae96..7e16ad9 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -1421,6 +1421,8 @@
   rtc::Thread* worker_thread = nullptr;
   rtc::Thread* signaling_thread = nullptr;
   rtc::SocketFactory* socket_factory = nullptr;
+  // The `packet_socket_factory` will only be used if CreatePeerConnection is
+  // called without a `port_allocator`.
   std::unique_ptr<rtc::PacketSocketFactory> packet_socket_factory;
   std::unique_ptr<TaskQueueFactory> task_queue_factory;
   std::unique_ptr<cricket::MediaEngineInterface> media_engine;