mention we don't keep the 16-bit ST12 synchronization word

Author: robUx4

cellar-codec/block_additional_mappings/smpte-st12-1-timecode.md

@@ -6,7 +6,7 @@ If the Block uses Lacing, the timecode value is associated with the first frame
 
 The `Block Additional Mapping` contains a full binary representation of a 64-bit SMPTE timecode
 value stored in big-endian format and expressed exactly as defined in Section 8 and 9
-of SMPTE 12M [@!SMPTE.ST12-1].
+of SMPTE 12M [@!SMPTE.ST12-1], without the 16-bit synchronization word.
 For convenience, here are the time address bit assignments as described in [@?RFC5484, section 6.2]:
 
 | Bit Positions | Label                  |