Always mention Box::pin when dealing with !Unpin

Author: estebank

core/src/marker.rs

@@ -765,7 +765,7 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 /// [`pin` module]: crate::pin
 #[stable(feature = "pin", since = "1.33.0")]
 #[rustc_on_unimplemented(
-    on(_Self = "std::future::Future", note = "consider using `Box::pin`",),
+    note = "consider using `Box::pin`",
     message = "`{Self}` cannot be unpinned"
 )]
 #[lang = "unpin"]