Update the "gradual guarantee" section in the docs

[sharkdp]

The example in that section still works, but it might be worth checking if that section (and the title) imply a wider support for the gradual guarantee than what we provide, given some recent changes. At the very least, the "info" box should be updated.

https://docs.astral.sh/ty/features/type-system/#gradual-guarantee

[sharkdp]

https://docs.astral.sh/ty/reference/typing-faq/#what-is-the-unknown-type-and-when-does-it-appear might also need an update.

[carljm]

Yes, I was planning to do this today; thank you!