* Fixes the 20+ TC items' discounts being applied incorrectly (#73597)
## About The Pull Request
When #22429 was added, the author's expressed intention was to make 20+
TC items discount less. However, by accident, it has been instead
doubled. I have fixed this problem, while also inverting the values in
`get_discount` to make the calculations easier, and make it clearer what
the return value of the proc is for.
This way, if the 140 TC mauler rolls 75% off, it will be 38% off instead
for 87.5 TC, instead of 87% off for 17 TC.
Also autodocced some procs.
## Why It's Good For The Game
Makes the code work as originally intended.
## Changelog
🆑
fix: 20+ item's discounts are properly smaller, instead of being larger
/🆑
* Fixes the 20+ TC items' discounts being applied incorrectly
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Co-authored-by: Profakos <profakos@gmail.com>
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