Document that and why subtitle decoders do not support direct-rendering.

Originally committed as revision 24177 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3542,6 +3542,10 @@ attribute_deprecated int avcodec_decode_subtitle(AVCodecContext *avctx, AVSubtit
  * Return a negative value on error, otherwise return the number of bytes used.
  * If no subtitle could be decompressed, got_sub_ptr is zero.
  * Otherwise, the subtitle is stored in *sub.
+ * Note that CODEC_CAP_DR1 is not available for subtitle codecs. This is for
+ * simplicity, because the performance difference is expect to be negligible
+ * and reusing a get_buffer written for video codecs would probably perform badly
+ * due to a potentially very different allocation pattern.
  *
  * @param avctx the codec context
  * @param[out] sub The AVSubtitle in which the decoded subtitle will be stored.