add UNBREAKABLE documentation to JSON_FLAGS.md

[MrDraMaster]

Summary

None

Purpose of change

UNBREAKABLE wasn't documented but exists, so it should be documented.

Describe the solution

Make sure UNBREAKABLE does what it implies.

Describe alternatives you've considered

Testing

Additional context

flag_UNBREAKABLE gets used in three C++ functions:

1. item.cpp's item::mod_damage, where it just returns FALSE (so no direct item damage)
2. item.cpp's item::damage_armor_durability, where it just returns FALSE (so no damage from armor wear)
3. melee.cpp's Character::handle_melee_wear, where it just returns FALSE (so no damage from smacking something on the head with it)