Improve documentation for unreachable

Fixes #18876
I see a few problems with this example:

- The function name is misspelled: it should read `get_at_least_100`.
- The function should return `u32`, not `()`.
- `sum` is never declared (`let mut sum = 0;` at the start of the function should work).
- Perhaps most importantly, the `unreachable!()` here is unnecessary (it throws a warning for unreachable code) and can be removed, as the compiler can already verify that the only way the `loop` can be exited (besides panicking) is with a `return`.