Clarify AVFMT_TS_DISCONT and muxers.

Originally committed as revision 26089 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -320,7 +320,7 @@ typedef struct AVFormatParameters {
 #define AVFMT_GLOBALHEADER  0x0040 /**< Format wants global header. */
 #define AVFMT_NOTIMESTAMPS  0x0080 /**< Format does not need / have any timestamps. */
 #define AVFMT_GENERIC_INDEX 0x0100 /**< Use generic index building code. */
-#define AVFMT_TS_DISCONT    0x0200 /**< Format allows timestamp discontinuities. */
+#define AVFMT_TS_DISCONT    0x0200 /**< Format allows timestamp discontinuities. Note, muxers always require valid (monotone) timestamps */
 #define AVFMT_VARIABLE_FPS  0x0400 /**< Format allows variable fps. */
 #define AVFMT_NODIMENSIONS  0x0800 /**< Format does not need width/height */