Document AVCodecContext channels field.

patch by Stefano Sabatini, stefano.sabatini-lala poste it

Originally committed as revision 13115 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -921,7 +921,7 @@ typedef struct AVCodecContext {
 
     /* audio only */
     int sample_rate; ///< samples per second
-    int channels;
+    int channels;    ///< number of audio channels
 
     /**
      * audio sample format