docs: show using the ? operator with different functions

Signed-off-by: Nathaniel McCallum <nathaniel@profian.com>

axum-core/src/response/mod.rs

@@ -36,9 +36,17 @@ pub type Response<T = BoxBody> = http::Response<T>;
 ///     http::StatusCode,
 /// };
 ///
+/// fn foo() -> Result<(), (StatusCode, &'static str)> {
+///     Err((StatusCode::NOT_FOUND, "not found"))
+/// }
+///
+/// fn bar() -> Result<(), StatusCode> {
+///     Err(StatusCode::BAD_REQUEST)
+/// }
+///
 /// fn handler() -> Result<&'static str> {
-///     Err((StatusCode::NOT_FOUND, "not found"))?;
-///     Err(StatusCode::BAD_REQUEST)?;
+///     foo()?;
+///     bar()?;
 ///     Ok("ok")
 /// }
 /// ```