Improved comment for hardcore mode

- Added explanation of effect
- Moved to 'General' section instead of 'Skills'
Fixed crash when placing block
A little clean-up in EntityArrowCustom

Prefer to use parent fields when possible, so removed duplicates:
- tile coordinates were made public by Forge
- damage has both a getter and a setter

Changes to write/read NBT:
- no longer call super method, as it did not call its super method
anyway and we don't need to write each entry twice
- added remaining fields from EntityArrow so that they are saved /
loaded