Document the fact that debug_handler doesn't work within impl blocks (#1800)

Co-authored-by: David Pedersen <david.pdrsn@gmail.com>

axum-macros/src/lib.rs

@@ -540,6 +540,32 @@ pub fn derive_from_request_parts(item: TokenStream) -> TokenStream {
 /// }
 /// ```
 ///
+/// # Limitations
+///
+/// This macro does not work for associated functions — functions defined in an `impl` block that
+/// don't take `self`:
+///
+/// ```compile_fail
+/// use axum::{debug_handler, extract::Path};
+///
+/// struct App {}
+///
+/// impl App {
+///     #[debug_handler]
+///     async fn handler(Path(_): Path<String>) {}
+/// }
+/// ```
+///
+/// This will yield an error similar to this:
+///
+/// ```text
+/// error[E0425]: cannot find function `__axum_macros_check_handler_0_from_request_check` in this scope
+//    --> src/main.rs:xx:xx
+//     |
+//  xx |     pub async fn handler(Path(_): Path<String>)  {}
+//     |                                   ^^^^ not found in this scope
+/// ```
+///
 /// # Performance
 ///
 /// This macro has no effect when compiled with the release profile. (eg. `cargo build --release`)