Add enum value "track" to bordertype

add docs so that meaning is clear, also add docs to track so that the usage is clear

Links

• Forum discussion:
  • Vasco Kolmorgen, 11.10.2024: https://www.railml.org/forum/index.php?t=msg&th=984&goto=3335&#msg_3335
• Git tickets:
  • #579 (closed)
• Wiki documentation:
  • IS:border: https://wiki3.railml.org/wiki/IS:border