Better example for UnionToIntersection

[boenrobot]

The example about UnionToIntersection seems really specific and it doesn't quite make it clear how the resulting "Fn" will be used... Heck, even the source docs say "#darkmagic".

Could anyone please demystify it just a little? At least finish the example to show how Fn will be used, and when will it error.

[quezak]

There is actually one usecase being proposed as a separate useful type in #100, though it's not about functions -- @krzkaczor could you describe your original case for this type?

[Beraliv]

There are good several solutions in type-challenges:

• 55 - Union to Intersection type-challenges/type-challenges#775
• 55 - Union to Intersection type-challenges/type-challenges#1051

But in repo we should have more interesting examples