Add doc tests to Panic documentation

We recently added these docs:

https://github.com/killercup/a-range/blob/ce391ed856e5fb52e9445496ccec046b91359ed5/src/lib.rs#L54-L58

This behavior is tested in this unit test:

https://github.com/killercup/a-range/blob/ce391ed856e5fb52e9445496ccec046b91359ed5/src/lib.rs#L393-L397

It is also possible for this to become a [doc test](https://doc.rust-lang.org/1.29.2/rustdoc/documentation-tests.html#attributes) (as I've just found out), which would make this more visible.

- [ ] Turn `fail_up_to_invalid_range` and `fail_down_to_invalid_range` tests into doc tests

Start in src/lib.rs at the Panic documentation around lines 54-58 and the unit tests around lines 393-397. Review the fail_up_to_invalid_range and fail_down_to_invalid_range tests, then run the Rust documentation tests to verify both cases are covered as doc tests.

Written by the indexing model from the issue text.