mention in macro docs

crates/wiggle/macro/src/lib.rs

@@ -39,6 +39,10 @@ use syn::parse_macro_input;
 ///   `{ errno => YourErrnoType }`. This allows you to use the `UserErrorConversion`
 ///   trait to map these rich errors into the flat witx type, or to terminate
 ///   WebAssembly execution by trapping.
+///     * Instead of requiring the user to define an error type, wiggle can
+///       generate an error type for the user which has conversions to/from
+///       the base type, and permits trapping, using the syntax
+///       `errno => trappable AnErrorType`.
 /// * Optional: `async` takes a set of witx modules and functions which are
 ///   made Rust `async` functions in the module trait.
 ///