message: Add type-safe unwrap example
diff: ///
  /// Once the ! type is stable, this is not needed anymore.
  /// See <https://github.com/rust-lang/rust/issues/35121>.
  +///
  +/// ## Examples
  +///
  +/// When using `Never` in a `Result`, we can unwrap in a type-safe way:
  +///
  +/// ```
  +/// use cosmwasm_std::Never;
  +///
  +/// pub fn safe_unwrap<T>(res: Result<T, Never>) -> T {
  +/// match res {
  +/// Ok(value) => value,
  +/// Err(err) => match err {},
  +/// }
  +/// }
  +///
  +/// let res: Result<i32, Never> = Ok(5);
  +/// assert_eq!(safe_unwrap(res), 5);
  +/// ```
  pub enum Never {}
  // The Debug implementation is needed to allow the use of `Result::unwrap`.
  