shop: document some constants s.t. object events don't break

include/shop.h

@@ -41,7 +41,7 @@ struct ShopData
     /*0x200B*/ u8 scrollIndicatorsTaskId;
     /*0x200C*/ u8 iconSlot;
     /*0x200D*/ u8 itemSpriteIds[2];
-    /*0x2010*/ s16 viewportObjects[16][5];
+    /*0x2010*/ s16 viewportObjects[OBJECT_EVENTS_COUNT][5];
 };
 
 void CreatePokemartMenu(const u16 *);