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

Many of these interfaces are not intuitive, or are the way they are for
complex historical reasons, so it would be nice to document these things
for future developers.

Also, many nonstandard things (such as RTCConfiguration options) were
not documented at all before this CL.

BUG=webrtc:7131
TBR=pthatcher@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2680273002
Cr-Commit-Position: refs/heads/master@{#16485}

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 {