Undocumented restriction on LuaEquipment.shield

[half a cat]

For equipment that is not shield equipment, writing to `LuaEquipment.shield` will abort the game with error message "Equipment is not shields.". Please add this restriction to the API documentation (https://lua-api.factorio.com/latest/cla ... tml#shield). I believe that the following description would be correct: This field is only writable if the type is `energy-shield-equipment`.