Clarify middleware::from_fn argument requirements (#2834)

axum/src/middleware/from_fn.rs

@@ -19,9 +19,10 @@ use tower_service::Service;
 /// `from_fn` requires the function given to
 ///
 /// 1. Be an `async fn`.
-/// 2. Take one or more [extractors] as the first arguments.
-/// 3. Take [`Next`](Next) as the final argument.
-/// 4. Return something that implements [`IntoResponse`].
+/// 2. Take zero or more [`FromRequestParts`] extractors.
+/// 3. Take exactly one [`FromRequest`] extractor as the second to last argument.
+/// 4. Take [`Next`](Next) as the last argument.
+/// 5. Return something that implements [`IntoResponse`].
 ///
 /// Note that this function doesn't support extracting [`State`]. For that, use [`from_fn_with_state`].
 ///