Neither really addresses the issue. Making a type for a single kind of string seems like an abuse of types just to shoehorn the documentation in. Documentation can be used directly of course, but that moots all of this -- just document? Yeah, but naming the variable is super quick compared to either.
Variable names is just documentation. Having types that can assert some condition on the underlying value is not even comparable to "having a named return variable". (just document? just name variables?) You don't care about what the name of the returned value is, you care about what *it is*.
But in native languages like C/C++/etc, this pattern usually hurts performance. Separate arguments go into registers (faster but depends on number of arguments), while structs often involve indirection, and sometimes alignment/padding issues. Also, passing a const struct doesn’t guarantee the fields inside are truly const.
That said, it's context dependent. If you're passing stuff through multiple layers or want a more stable function signature, using a struct can be cleaner. Just not something I’d generalize to any language or any use case.