Updated bone_drop configuration description

* Added a note to state the default item's trade restrictions.

conf/battle/player.conf

@@ -135,6 +135,8 @@ friend_auto_add: yes
 invite_request_check: yes
 
 // Players' will drop a 'Skull' when killed?
+// Note: The 'Skull' item (ID 7420) is trade restricted by default. Please check
+// the item_trade database to adjust accordingly.
 // 0 = Disabled
 // 1 = Dropped only in PvP maps
 // 2 = Dropped in all situations