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

webrtc/api/mediaconstraintsinterface.h

Index: webrtc/api/mediaconstraintsinterface.h
diff --git a/webrtc/api/mediaconstraintsinterface.h b/webrtc/api/mediaconstraintsinterface.h
index 2017890f62cd54430722aa24118d4517f60d1078..8dafea405eaedc4b71e414de0c6dc4103d74168f 100644
--- a/webrtc/api/mediaconstraintsinterface.h
+++ b/webrtc/api/mediaconstraintsinterface.h
@@ -28,9 +28,12 @@
 
 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 {