lavu/timecode: clarify error msg for timecode_rate

The user-supplied value for timecode_rate in drawtext is rounded
to nearest integer. So, a supplied value of 0.49 or lower is rounded to 0.
This throws a misleading error message which says "Timecode frame rate must be
specified". Changed message to account for values under one.

Also noted supported framerates for drop TC.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -7365,7 +7365,9 @@ format. It can be used with or without text parameter. @var{timecode_rate}
 option must be specified.
 
 @item timecode_rate, rate, r
-Set the timecode frame rate (timecode only).
+Set the timecode frame rate (timecode only). Value will be rounded to nearest
+integer. Minimum value is "1".
+Drop-frame timecode is supported for frame rates 30 & 60.
 
 @item tc24hmax
 If set to 1, the output of the timecode option will wrap around at 24 hours.

libavutil/timecode.c

@@ -155,7 +155,7 @@ static int check_fps(int fps)
 static int check_timecode(void *log_ctx, AVTimecode *tc)
 {
     if ((int)tc->fps <= 0) {
-        av_log(log_ctx, AV_LOG_ERROR, "Timecode frame rate must be specified\n");
+        av_log(log_ctx, AV_LOG_ERROR, "Valid timecode frame rate must be specified. Minimum value is 1\n");
         return AVERROR(EINVAL);
     }
     if ((tc->flags & AV_TIMECODE_FLAG_DROPFRAME) && tc->fps != 30 && tc->fps != 60) {