hashes: Put attribute under rustdoc

Like we do for all the other macros ptu the `macro_export` attribute
below the macro comments.

hashes/src/macros.rs

@@ -2,8 +2,8 @@
 
 //! Public macros.
 
-#[macro_export]
 /// Adds hexadecimal formatting implementation of a trait `$imp` to a given type `$ty`.
+#[macro_export]
 macro_rules! hex_fmt_impl(
     ($reverse:expr, $len:expr, $ty:ident) => (
         $crate::hex_fmt_impl!($reverse, $len, $ty, );