update comment in an example

portable-atomic-util/src/arc.rs

@@ -1520,7 +1520,7 @@ impl Arc<dyn Any + Send + Sync> {
     /// let my_string = "Hello World".to_string();
     /// print_if_string(Arc::from(Box::new(my_string) as Box<dyn Any + Send + Sync>));
     /// print_if_string(Arc::from(Box::new(0i8) as Box<dyn Any + Send + Sync>));
-    /// // or with --cfg portable_atomic_unstable_coerce_unsized:
+    /// // or with --cfg portable_atomic_unstable_coerce_unsized in RUSTFLAGS (only for Rust nightly):
     /// // print_if_string(Arc::new(my_string));
     /// // print_if_string(Arc::new(0i8));
     /// ```