[DOCS] Document runway-specific top-down

[kristiankunc]

Runway-specific top-down is currently undocumented on the wiki.
Current documentation:

topdown - Optional Array of Strings - ID of owning positions (Above TWR, in descending order of priority) in top-level positions array.

But as seen in /data/lo.json:5419, this is not always followed. I assume this is a very rare edge case but makes the data inconsistent and should be at least document if not implemented differently for consistency.