Bug 1506884 - Document thread usage of mRtpPacketQueue in VideoConduit; r=padenot

This was added during the branch 64 update, but was not documented at that time.

Differential Revision: https://phabricator.services.mozilla.com/D25054

--HG--
extra : rebase_source : 49d7be8d1db97edcd601ff3a0e502521c21e87ad
extra : source : 6c4544bfbe7262555ca7154f3367ad24d7b55669
extra : histedit_source : baa8da9d12766ff9ea133aed356832f273de8431

media/webrtc/signaling/src/media-conduit/VideoConduit.h

@@ -591,6 +591,7 @@ class WebrtcVideoConduit
   // and when receiving packets (sts).
   Atomic<uint32_t> mRecvSSRC;  // this can change during a stream!
 
+  // Accessed only on mStsThread.
   RtpPacketQueue mRtpPacketQueue;
 
   // The lifetime of these codecs are maintained by the VideoConduit instance.