Add some help for the analyzeduration option.

patch by Víctor Paesa, wzrlpy arsystel com

Originally committed as revision 8631 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -296,7 +296,7 @@ static const AVOption options[]={
 {"genpts", "generate pts", 0, FF_OPT_TYPE_CONST, AVFMT_FLAG_GENPTS, INT_MIN, INT_MAX, D, "fflags"},
 {"track", " set the track number", OFFSET(track), FF_OPT_TYPE_INT, DEFAULT, 0, INT_MAX, E},
 {"year", "set the year", OFFSET(year), FF_OPT_TYPE_INT, DEFAULT, INT_MIN, INT_MAX, E},
-{"analyzeduration", NULL, OFFSET(max_analyze_duration), FF_OPT_TYPE_INT, 3*AV_TIME_BASE, 0, INT_MAX, D},
+{"analyzeduration", "how many microseconds are analyzed to estimate duration", OFFSET(max_analyze_duration), FF_OPT_TYPE_INT, 3*AV_TIME_BASE, 0, INT_MAX, D},
 {NULL},
 };