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/rtpreceiverinterface.h

@@ -54,6 +54,7 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
   // but this API also applies them to receivers, similar to ORTC:
   // http://ortc.org/wp-content/uploads/2016/03/ortc.html#rtcrtpparameters*.
   virtual RtpParameters GetParameters() const = 0;
+  // Currently, doesn't support changing any parameters, but may in the future.
   virtual bool SetParameters(const RtpParameters& parameters) = 0;
 
   // Does not take ownership of observer.
@@ -65,6 +66,8 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
 };
 
 // Define proxy for RtpReceiverInterface.
+// TODO(deadbeef): Move this to .cc file and out of api/. What threads methods
+// are called on is an implementation detail.
 BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
   PROXY_SIGNALING_THREAD_DESTRUCTOR()
   PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)