Add comments and direction_reverse function

Add a bit of Javadocs, and introduce helper function for reversing directions because the ^2 trick used elsewhere in the codebase is not immediately obvious exactly what it does.

src/openrct2/world/Location.hpp

@@ -129,8 +129,26 @@ struct TileCoordsXYZ
     int32_t x = 0, y = 0, z = 0;
 };
 
+/**
+ * Cardinal directions are represented by the Direction type. It has four
+ * possible values:
+ * 0 is X-decreasing
+ * 1 is Y-increasing
+ * 2 is X-increasing
+ * 3 is Y-decreasing
+ * Direction is not used to model up/down, or diagonal directions.
+ */
 typedef uint8_t Direction;
 
+/**
+ * Given a direction, return the direction that points the other way,
+ * on the same axis.
+ */
+inline Direction direction_reverse(const Direction& dir)
+{
+    return dir ^ 2;
+}
+
 struct CoordsXYZD
 {
     int32_t x, y, z;