1) Add `node` and `node_pos` field for the `drown` field (analog to `node_damage` type)
2) Add `custom_type` field to allow a standardized way to report more details of the HP change
3) Make `builtin` report a `custom_type` for the HP changes caused by the `/kill` command and `core.do_item_eat`
4) Move the documentation of `PlayerHPChangeReason` to its own section
5) Fill in missing information of said documentation