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

webrtc/api/dtmfsenderinterface.h

Index: webrtc/api/dtmfsenderinterface.h
diff --git a/webrtc/api/dtmfsenderinterface.h b/webrtc/api/dtmfsenderinterface.h
index a4d270d905599de790ed52ed6aa4b621bc0b92f0..a1ef3ecec6ea2c0f0d1e3c165d7ce7f818493d81 100644
--- a/webrtc/api/dtmfsenderinterface.h
+++ b/webrtc/api/dtmfsenderinterface.h
@@ -16,12 +16,11 @@
 #include "webrtc/api/mediastreaminterface.h"
 #include "webrtc/base/refcount.h"
 
-// This file contains interfaces for DtmfSender.
-
 namespace webrtc {
 
-// DtmfSender callback interface. Application should implement this interface
-// to get notifications from the DtmfSender.
+// DtmfSender callback interface, used to implement RTCDtmfSender events.
+// Applications should implement this interface to get notifications from the
+// DtmfSender.
 class DtmfSenderObserverInterface {
  public:
   // Triggered when DTMF |tone| is sent.
@@ -35,13 +34,18 @@ class DtmfSenderObserverInterface {
 
 // The interface of native implementation of the RTCDTMFSender defined by the
 // WebRTC W3C Editor's Draft.
+// See: https://www.w3.org/TR/webrtc/#peer-to-peer-dtmf
 class DtmfSenderInterface : public rtc::RefCountInterface {
  public:
+  // Used to receive events from the DTMF sender. Only one observer can be
+  // registered at a time. UnregisterObserver should be called before the
+  // observer object is destroyed.
   virtual void RegisterObserver(DtmfSenderObserverInterface* observer) = 0;
   virtual void UnregisterObserver() = 0;
 
-  // Returns true if this DtmfSender is capable of sending DTMF.
-  // Otherwise returns false.
+  // Returns true if this DtmfSender is capable of sending DTMF. Otherwise
+  // returns false. To be able to send DTMF, the associated RtpSender must be
+  // able to send packets, and a "telephone-event" codec must be negotiated.
   virtual bool CanInsertDtmf() = 0;
 
   // Queues a task that sends the DTMF |tones|. The |tones| parameter is treated
@@ -49,20 +53,26 @@ class DtmfSenderInterface : public rtc::RefCountInterface {
   // * generate the associated DTMF tones. The characters a to d are equivalent
   // to A to D. The character ',' indicates a delay of 2 seconds before
   // processing the next character in the tones parameter.
+  //
   // Unrecognized characters are ignored.
+  //
   // The |duration| parameter indicates the duration in ms to use for each
-  // character passed in the |tones| parameter.
-  // The duration cannot be more than 6000 or less than 70.
-  // The |inter_tone_gap| parameter indicates the gap between tones in ms.
-  // The |inter_tone_gap| must be at least 50 ms but should be as short as
+  // character passed in the |tones| parameter. The duration cannot be more
+  // than 6000 or less than 70.
+  //
+  // The |inter_tone_gap| parameter indicates the gap between tones in ms. The
+  // |inter_tone_gap| must be at least 50 ms but should be as short as
   // possible.
+  //
   // If InsertDtmf is called on the same object while an existing task for this
   // object to generate DTMF is still running, the previous task is canceled.
   // Returns true on success and false on failure.
   virtual bool InsertDtmf(const std::string& tones, int duration,
                           int inter_tone_gap) = 0;
 
-  // Returns the track given as argument to the constructor.
+  // Returns the track given as argument to the constructor. Only exists for
+  // backwards compatibilty; now that DtmfSenders are tied to RtpSenders, it's
+  // no longer relevant.
   virtual const AudioTrackInterface* track() const = 0;
 
   // Returns the tones remaining to be played out.