lavf/mpegtsenc: clarify pcr_period unit of measurement

pcr_period is in milliseconds.
Signed-off-by: Lou Logan <lou@lrcd.com>

libavformat/mpegtsenc.c

@@ -1945,7 +1945,7 @@ static const AVOption options[] = {
     { "omit_video_pes_length", "Omit the PES packet length for video packets",
       offsetof(MpegTSWrite, omit_video_pes_length), AV_OPT_TYPE_BOOL,
       { .i64 = 1 }, 0, 1, AV_OPT_FLAG_ENCODING_PARAM },
-    { "pcr_period", "PCR retransmission time",
+    { "pcr_period", "PCR retransmission time in milliseconds",
       offsetof(MpegTSWrite, pcr_period), AV_OPT_TYPE_INT,
       { .i64 = PCR_RETRANS_TIME }, 0, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM },
     { "pat_period", "PAT/PMT retransmission time limit in seconds",