add documentation on timecode_stream @id

[dericed]

The documentation is here https://github.com/MediaArea/timecodexml/blob/main/MediaTimecode.xsd#L56C29-L58.

20260106-mkv_tc6-_deck2ffv1.mkv.zip

With the sample above the stream id's are:

  <timecode_stream id="1-101" format="matroska_blockadditional" frame_rate="29.970" frame_count="4">
...
  <timecode_stream id="1-102" format="matroska_blockadditional" frame_rate="29.970" frame_count="4">

The id mixes the stream id of the associated track (video, if counting starting at 1) and the blockadditionalmapping id (101 and 102). Please add documentation that in some cases two values are crammed into one field.