Updates NPC_SIEGEMODE comment to known behavior (#7327)

src/map/skill.cpp

@@ -9676,7 +9676,7 @@ int skill_castend_nodamage_id (struct block_list *src, struct block_list *bl, ui
 		break;
 
 	case NPC_SIEGEMODE:
-		// not sure what it does
+		// Not implemented/used: Gives EFST_SIEGEMODE which reduces speed to 1000.
 		clif_skill_nodamage(src,bl,skill_id,skill_lv,1);
 		break;