Add comment/code documentation

sfsmcd/src/main.rs

@@ -55,6 +55,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     // Will (afaik) be stabilized in Rust 1.48
     // core::future::pending::<()>().await;
+
+    // Wait until SIGTERM (when systemd stops this service), then shutdown
     tokio::signal::unix::signal(tokio::signal::unix::SignalKind::terminate())?.recv().await;
     info!("Got SIGTERM, shutting down");
     Ok(())