AudioDecoder: Document that the sample rate and number of channels is constant

Review-Url: https://codereview.webrtc.org/2029273003
Cr-Commit-Position: refs/heads/master@{#13012}

webrtc/modules/audio_coding/codecs/audio_decoder.h

@@ -93,7 +93,8 @@ class AudioDecoder {
   // Returns true if the packet has FEC and false otherwise.
   virtual bool PacketHasFec(const uint8_t* encoded, size_t encoded_len) const;
 
-  // Returns the actual sample rate of the decoder's output.
+  // Returns the actual sample rate of the decoder's output. This value may not
+  // change during the lifetime of the decoder.
   // NOTE: For now, this has a default implementation that returns an unusable
   // value (-1). That default implementation will go away soon, and at the same
   // time callers will start relying on the return value, so make sure you
@@ -101,6 +102,8 @@ class AudioDecoder {
   // TODO(kwiberg): Remove the default implementation.
   virtual int SampleRateHz() const;
 
+  // The number of channels in the decoder's output. This value may not change
+  // during the lifetime of the decoder.
   virtual size_t Channels() const = 0;
 
  protected: