add comments about 5v3 check diff

Author: James Heppell

src/fen.ts

@@ -117,7 +117,7 @@ export function parseRemainingChecks(part: string): Result<RemainingChecks, FenE
     const black = parseSmallUint(parts[2]);
     if (!defined(white) || white > 5 || !defined(black) || black > 5)
       return Result.err(new FenError(InvalidFen.RemainingChecks));
-    return Result.ok(new RemainingChecks(5 - white, 5 - black)); //TODO how to know about 5check vs 3check variant to set accordingly
+    return Result.ok(new RemainingChecks(5 - white, 5 - black)); //Old notation counting down, not used in games, therefore using highest check mode 5
   } else if (parts.length === 2) {
     const white = parseSmallUint(parts[0]);
     const black = parseSmallUint(parts[1]);