avcodec: clarify documentation of CODEC_CAP_DELAY

libavcodec/avcodec.h

@@ -674,8 +674,10 @@ typedef struct RcOverride{
 /* Codec can export data for HW decoding (XvMC). */
 #define CODEC_CAP_HWACCEL         0x0010
 /**
- * Codec has a nonzero delay and needs to be fed with NULL at the end to get the delayed data.
- * If this is not set, the codec is guaranteed to never be fed with NULL data.
+ * Codec has a nonzero delay and needs to be fed with avpkt->data=NULL,
+ * avpkt->size=0 at the end to get the delayed data until the decoder no longer
+ * returns frames. If this is not set, the codec is guaranteed to never be fed
+ * with NULL data.
  */
 #define CODEC_CAP_DELAY           0x0020
 /**