message: Improve Never documentation
diff: /// where we want to ensure that no error is returned, such as
  /// the `ibc_packet_receive` entry point.
  ///
  +/// In contrast to `Empty`, this does not have a JSON schema
  +/// and cannot be used for message and query types.
  +///
  /// Once the ! type is stable, this is not needed anymore.
  /// See <https://github.com/rust-lang/rust/issues/35121>.
  pub enum Never {}
  +// The Debug implementation is needed to allow the use of `Result::unwrap`.
  impl core::fmt::Debug for Never {
  fn fmt(&self, _f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
  // This is unreachable because no instance of Never can exist
  @@ -13,6 +17,8 @@ impl core::fmt::Debug for Never {
  }
  }
  +// The Display implementation is needed to fulfill the ToString requirement of
  +// entry point errors: `Result<IbcReceiveResponse<C>, E>` with `E: ToString`.
  impl core::fmt::Display for Never {
  fn fmt(&self, _f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
  // This is unreachable because no instance of Never can exist
  