clarify doc on io module

src/io.rs

@@ -1,3 +1,3 @@
-//! Re-exports of std values for when the std is available.
+//! Re-exports of std values for when the std is available or local reimplementations if std is not available
 #[cfg(feature = "std")]
 pub use std::io::{Error, ErrorKind, Read, Write};