docs(iroh-net): Document cfg(test) items as well (#2819)

## Description

It seems the newer rust release enforces missing docs on this as well.


## Breaking Changes

None

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
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## Change checklist

- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.

iroh-net/src/relay/server/streams.rs

@@ -92,6 +92,7 @@ pub enum MaybeTlsStream {
     Plain(tokio::net::TcpStream),
     /// A Tls wrapped [`tokio::net::TcpStream`]
     Tls(tokio_rustls::server::TlsStream<tokio::net::TcpStream>),
+    /// An in-memory bidirectional pipe.
     #[cfg(test)]
     Test(tokio::io::DuplexStream),
 }