Move no_std above comment

Currently the no_std attribute is below a comment that does not relate
to it. For slightly improved clarity move the attribute above the
comment.

io/src/lib.rs

@@ -13,9 +13,9 @@
 //! This traits are not one-for-one drop-ins, but are as close as possible while still implementing
 //! `std::io`'s traits without unnecessary complexity.
 
+#![cfg_attr(not(feature = "std"), no_std)]
 // Experimental features we need.
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
-#![cfg_attr(not(feature = "std"), no_std)]
 
 #[cfg(any(feature = "alloc", feature = "std"))]
 extern crate alloc;