Adding more comments to every header file in api/ subdirectory.

webrtc/api/mediaconstraintsinterface.h

 /*
  *  Copyright 2013 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 10 matching lines @@
 #define WEBRTC_API_MEDIACONSTRAINTSINTERFACE_H_
 
 #include <string>
 #include <vector>
 
 #include "webrtc/base/optional.h"
 #include "webrtc/api/peerconnectioninterface.h"
 
 namespace webrtc {
 
-// MediaConstraintsInterface
 // Interface used for passing arguments about media constraints
 // to the MediaStream and PeerConnection implementation.
+//
+// Constraints may be either "mandatory", which means that unless satisfied,
+// the method taking the constraints should fail, or "optional", which means
+// they may not be satisfied..
 class MediaConstraintsInterface {
  public:
   struct Constraint {
     Constraint() {}
     Constraint(const std::string& key, const std::string value)
         : key(key), value(value) {
     }
     std::string key;
     std::string value;
   };
@@ skipping 98 matching lines @@
     PeerConnectionInterface::RTCConfiguration* configuration);
 
 // Copy all relevant constraints into an AudioOptions object.
 void CopyConstraintsIntoAudioOptions(
     const MediaConstraintsInterface* constraints,
     cricket::AudioOptions* options);
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_MEDIACONSTRAINTSINTERFACE_H_