message: Improve unimpl! docs
diff: }
  // Just public, because of the unimpl macro
  +#[doc(hidden)]
  pub struct NonFatal(pub String);
  +/// Use when something in the current function is unimplemented.
  +///
  +/// This will emit an error and continue codegen at a different function.
  pub macro unimpl($($tt:tt)*) {
  panic!(NonFatal(format!($($tt)*)));
  }
  