Clarify bincode encoding in testing docs

wireframe_testing/src/lib.rs

@@ -3,8 +3,11 @@
 //!
 //! These helpers spawn the application on a `tokio::io::duplex` stream and
 //! return all bytes written by the app for easy assertions. They work with any
-//! message implementing [`serde::Serialize`] – the example uses a simple `u8`
-//! value so no generics are required.
+//! message implementing [`serde::Serialize`]. The payload is encoded with
+//! [`bincode::encode_to_vec`] using [`bincode::config::standard()`], which means
+//! little-endian byte order, variable-length integer encoding and no byte limit
+//! are applied. The example uses a simple `u8` value so no generics are
+//! required.
 //!
 //! ```rust
 //! use wireframe::app::WireframeApp;