Move help link to error index

compiler/rustc_error_codes/src/error_codes/E0373.md

@@ -50,3 +50,25 @@ fn foo() -> Box<Fn(u32) -> u32> {
 
 Now that the closure has its own copy of the data, there's no need to worry
 about safety.
+
+This error may also be encountered while using `async` blocks:
+
+```compile_fail,E0373
+use std::sync::Arc;
+use tokio::runtime::Runtime; // 0.3.1
+
+async fn f() {
+    let room_ref = Arc::new(Vec::new());
+
+    let gameloop_handle = Runtime::new().unwrap().spawn(async {
+        game_loop(Arc::clone(&room_ref))
+    });
+    gameloop_handle.await;
+}
+
+fn game_loop(v: Arc<Vec<usize>>) {}
+```
+
+Similarly to closures, `async` blocks are not executed immediately and may
+capture closed-over data by reference. For more information, see 
+https://rust-lang.github.io/async-book/03_async_await/01_chapter.html.