Slightly clean up the error for recursive async fn

* Make it clear that type erasure is required, not just pointer
  indirection.
* Don't make the message specific to direct recursion.

src/librustc_typeck/check/mod.rs

@@ -1420,8 +1420,8 @@ fn check_opaque_for_cycles<'tcx>(
                 tcx.sess, span, E0733,
                 "recursion in an `async fn` requires boxing",
             )
-            .span_label(span, "an `async fn` cannot invoke itself directly")
-            .note("a recursive `async fn` must be rewritten to return a boxed future.")
+            .span_label(span, "recursive `async fn`")
+            .note("a recursive `async fn` must be rewritten to return a boxed `dyn Future`.")
             .emit();
         } else {
             let mut err = struct_span_err!(

src/test/ui/async-await/recursive-async-impl-trait-type.stderr

@@ -2,9 +2,9 @@ error[E0733]: recursion in an `async fn` requires boxing
   --> $DIR/recursive-async-impl-trait-type.rs:5:40
    |
 LL | async fn recursive_async_function() -> () {
-   |                                        ^^ an `async fn` cannot invoke itself directly
+   |                                        ^^ recursive `async fn`
    |
-   = note: a recursive `async fn` must be rewritten to return a boxed future.
+   = note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`.
 
 error: aborting due to previous error