lavc: extend frame_size doxy.

libavcodec/avcodec.h

@@ -2081,7 +2081,13 @@ typedef struct AVCodecContext {
 
     /* The following data should not be initialized. */
     /**
-     * Samples per packet, initialized when calling 'init'.
+     * Number of samples per channel in an audio frame.
+     *
+     * - encoding: set by libavcodec in avcodec_open2(). Each submitted frame
+     *   except the last must contain exactly frame_size samples per channel.
+     *   May be 0 when the codec has CODEC_CAP_VARIABLE_FRAME_SIZE set, then the
+     *   frame size is not restricted.
+     * - decoding: may be set by some decoders to indicate constant frame size
      */
     int frame_size;