Adding comments about threading around CreatePeerConnectionFactory.

webrtc/api/peerconnectioninterface.h

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 616 matching lines @@
   virtual void StopRtcEventLog() = 0;
 
  protected:
   // Dtor and ctor protected as objects shouldn't be created or deleted via
   // this interface.
   PeerConnectionFactoryInterface() {}
   ~PeerConnectionFactoryInterface() {} // NOLINT
 };
 
 // Create a new instance of PeerConnectionFactoryInterface.
+//
+// This method relies on the thread it's called on as the "signaling thread"
+// for the PeerConnectionFactory it creates.
+//
+// As such, if the current thread is not already running an rtc::Thread message
+// loop, an application using this method must eventually either call
+// rtc::Thread::Current()->Run(), or call
+// rtc::Thread::Current()->ProcessMessages() within the application's own
+// message loop.
 rtc::scoped_refptr<PeerConnectionFactoryInterface>
 CreatePeerConnectionFactory();
 
 // Create a new instance of PeerConnectionFactoryInterface.
-// Ownership of |factory|, |default_adm|, and optionally |encoder_factory| and
-// |decoder_factory| transferred to the returned factory.
+//
+// |worker_thread| and |signaling_thread| are the only mandatory
+// parameters.
+//
+// If non-null, ownership of |default_adm|, |encoder_factory| and
+// |decoder_factory| are transferred to the returned factory.
 rtc::scoped_refptr<PeerConnectionFactoryInterface>
 CreatePeerConnectionFactory(
     rtc::Thread* worker_thread,
     rtc::Thread* signaling_thread,
     AudioDeviceModule* default_adm,
     cricket::WebRtcVideoEncoderFactory* encoder_factory,
     cricket::WebRtcVideoDecoderFactory* decoder_factory);
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_PEERCONNECTIONINTERFACE_H_