Document the CloseBattlePikeCurtain special

include/global.fieldmap.h

@@ -36,6 +36,11 @@ struct Tileset
     /*0x14*/ TilesetCB callback;
 };
 
+// Tilesets do not actually have s strict width.
+// This constant is simply used for the offset between rows of metatiles for
+// large tiles, such as the Battle Pike's curtain tile.
+#define TILESET_WIDTH 8
+
 struct MapLayout
 {
     /*0x00*/ s32 width;