Document RouterService (#1465)

Not sure what else there is to say :P

Fixes https://github.com/tokio-rs/axum/issues/1401

axum/src/routing/mod.rs

@@ -481,6 +481,10 @@ where
     }
 
     /// Convert this router into a [`RouterService`].
+    ///
+    /// # Panics
+    ///
+    /// Panics if the router was constructed with [`Router::inherit_state`].
     #[track_caller]
     pub fn into_service(self) -> RouterService<B> {
         RouterService::new(self)

axum/src/routing/service.rs

@@ -18,7 +18,7 @@ use crate::{
     response::Response,
 };
 
-/// TOOD: Docs
+/// A [`Router`] converted into a [`Service`].
 #[derive(Debug)]
 pub struct RouterService<B = Body> {
     routes: HashMap<RouteId, Route<B>>,
@@ -129,7 +129,6 @@ impl<B> Clone for RouterService<B> {
 impl<B> Service<Request<B>> for RouterService<B>
 where
     B: HttpBody + Send + 'static,
-    //S: Send + Sync + 'static,
 {
     type Response = Response;
     type Error = Infallible;