Document synchronous guest test isolation

src/session_tests.rs

@@ -97,6 +97,8 @@ async fn a_listing_reconnects_after_the_server_expires_the_control_session()
     })
     .await?
   });
+  // `Guest` is generated as a synchronous trait, so the repository rule for
+  // unavoidable blocking I/O requires isolating this native test call here.
   let client = tokio::task::spawn_blocking(move || -> TestResult {
     guest(Ftp::open("ftp".into(), config(port)))?;
     let first = guest(Ftp::list(Locator::Path("/".into()), None, 10))?;
@@ -137,6 +139,8 @@ async fn recovery_before_a_mutation_never_replays_a_lost_mutation_reply()
     })
     .await?
   });
+  // `Guest` is generated as a synchronous trait, so the repository rule for
+  // unavoidable blocking I/O requires isolating this native test call here.
   let client = tokio::task::spawn_blocking(move || -> TestResult {
     guest(Ftp::open("ftp".into(), config(port)))?;
     wait_expired.blocking_recv()?;