explain that Router::merge only merges paths and fallback (#2316)

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

axum/src/docs/routing/merge.md

@@ -1,4 +1,4 @@
-Merge two routers into one.
+Merge the paths and fallbacks of two routers into a single [`Router`].
 
 This is useful for breaking apps into smaller pieces and combining them
 into one.
@@ -71,6 +71,11 @@ let app = Router::new()
 # let _: axum::Router = app;
 ```
 
+# Merging routers with fallbacks
+
+When combining [`Router`]s with this method, the [fallback](Router::fallback) is also merged.
+However only one of the routers can have a fallback.
+
 # Panics
 
 - If two routers that each have a [fallback](Router::fallback) are merged. This

axum/src/routing/tests/merge.rs

@@ -135,6 +135,8 @@ async fn layer_and_handle_error() {
 
     let res = client.get("/timeout").send().await;
     assert_eq!(res.status(), StatusCode::REQUEST_TIMEOUT);
+    let res = client.get("/foo").send().await;
+    assert_eq!(res.status(), StatusCode::OK);
 }
 
 #[crate::test]