comment explaining that text subtitle need to be UTF-8 for now

Originally committed as revision 9592 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -262,7 +262,7 @@ enum CodecID {
     /* subtitle codecs */
     CODEC_ID_DVD_SUBTITLE= 0x17000,
     CODEC_ID_DVB_SUBTITLE,
-    CODEC_ID_TEXT,
+    CODEC_ID_TEXT,  /* raw UTF-8 text */
 
     CODEC_ID_MPEG2TS= 0x20000, /* _FAKE_ codec to indicate a raw MPEG-2 TS
                                 * stream (only used by libavformat) */