Added note about KeepAfterUse to pre-re item_db

- Follow-up to d4e0689
- Special thanks to Litro
- Also synchronized header comment of the item_db2

Signed-off-by: Haru <haru@dotalux.com>
Allowed flag values of map_count_oncell to stack

- Follow-up to ea34b80
- It is now possible to stack 0x1|0x2 as a valid flag value.

Signed-off-by: Haru <haru@dotalux.com>