doc: update documentation and tests for toUIntX functions

[TomasPuverle]

Update documentation on functions to reflect actual behavior.
Add tests to ensure said behavior is as documented.

Closes #5483

[TomasPuverle]

CC: @david-christiansen since I noticed most people ask you for documentation/wording review for PRs.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 56b78a0ed104c896c033bcd2284449cea2dd5d12 --onto 0196bca784f82f90b4efd2a85a400daf4ab767f8. (2024-09-27 17:25:53)

[david-christiansen]

Thanks for the ping! I was off away from the Internet for a few days. Other than marking code with backticks, I think the text is great :)

[david-christiansen]

(and the presence of tests even greater)

[TomasPuverle]

Thanks! I'll know to use backticks in the future.

As for the tests, I considered moving them to the format using #guard_msg but thought perhaps that was too big of a change for just a doc update. However, is it the general intention for all tests to be ported to the new "standard"?