Codechange: Make GetRail/RoadTypeInfoIndex() a member method.

This follows a similar pattern used for cargoes, houses and objects.

Also fixes incorrect documentation of GetRoadTypeInfoIndex().

src/road.cpp

@@ -23,6 +23,18 @@
 
 #include "safeguards.h"
 
+/**
+ * Get the RoadType for this RoadTypeInfo.
+ * @return RoadType in static RoadTypeInfo definitions.
+ */
+RoadType RoadTypeInfo::Index() const
+{
+	extern RoadTypeInfo _roadtypes[ROADTYPE_END];
+	size_t index = this - _roadtypes;
+	assert(index < ROADTYPE_END);
+	return static_cast<RoadType>(index);
+}
+
 /**
  * Return if the tile is a valid tile for a crossing.
  *