Clarifies the bandwidth estimation interfaces.

BUG=

Review URL: https://webrtc-codereview.appspot.com/965019

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3087 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/bitrate_controller/include/bitrate_controller.h b/webrtc/modules/bitrate_controller/include/bitrate_controller.h
index 002ab8f..82b7e4b 100644
--- a/webrtc/modules/bitrate_controller/include/bitrate_controller.h
+++ b/webrtc/modules/bitrate_controller/include/bitrate_controller.h
@@ -23,10 +23,12 @@
  /*
   * Observer class for the encoders, each encoder should implement this class
   * to get the target bitrate. It also get the fraction loss and rtt to
-  * optimize its settings for this type of network.
+  * optimize its settings for this type of network. |target_bitrate| is the
+  * target media/payload bitrate excluding packet headers, measured in bits
+  * per second.
   */
  public:
-  virtual void OnNetworkChanged(const uint32_t targer_bitrate,
+  virtual void OnNetworkChanged(const uint32_t target_bitrate,
                                 const uint8_t fraction_loss,  // 0 - 255.
                                 const uint32_t rtt) = 0;
 
@@ -46,6 +48,8 @@
 
   virtual RtcpBandwidthObserver* CreateRtcpBandwidthObserver() = 0;
 
+  // Gets the available payload bandwidth in bits per second. Note that
+  // this bandwidth excludes packet headers.
   virtual bool AvailableBandwidth(uint32_t* bandwidth) const = 0;
 
   /*