Add comment to distortion calculation

The formula for calculating the distortion seems wrong but practically it seems to work slightly better

core/src/main/java/com/google/zxing/qrcode/detector/FinderPatternFinder.java

@@ -695,6 +695,8 @@ public class FinderPatternFinder {
           // we need to check both two equal sides separately.
           // The value of |c^2 - 2 * b^2| + |c^2 - 2 * a^2| increases as dissimilarity
           // from isosceles right triangle.
+          // Heuristically it seems that the following formula works better (although it's
+          // not clear any more why...)
           double d = Math.abs(c - 2 * b) + Math.abs(c - 2 * a);
           if (d < distortion) {
             distortion = d;