Add information about the meaning of Router<S> to Router document…

…ation (#3235)

axum/src/routing/mod.rs

@@ -58,6 +58,12 @@ macro_rules! panic_on_err {
 pub(crate) struct RouteId(u32);
 
 /// The router type for composing handlers and services.
+///
+/// `Router<S>` means a router that is _missing_ a state of type `S` to be able
+/// to handle requests. Thus, only `Router<()>` (i.e. without missing state) can
+/// be passed to [`serve`]. See [`Router::with_state`] for more details.
+///
+/// [`serve`]: crate::serve()
 #[must_use]
 pub struct Router<S = ()> {
     inner: Arc<RouterInner<S>>,