Clarify network and test execution contracts

src/test_host.rs

@@ -1,5 +1,6 @@
-//! Native test transport. Guest calls run only inside `spawn_blocking`;
-//! production WASM always uses the host's sockets and network policy.
+//! Native test transport for the generated synchronous `Guest` API. It cannot
+//! await a Tokio socket, so tests confine it to `spawn_blocking` while their FTP
+//! server remains async. Production WASM always uses the host's async sockets.
 
 use std::cell::RefCell;
 use std::io::{Read, Write};