Put cfg attributes under rustdocs

By convention we put the compiler attributes underneath the rustdocs.

Author: tcharding

src/serde_macros.rs

@@ -15,8 +15,8 @@
 //! Macros for serde trait implementations, and supporting code.
 //!
 
-#[cfg(feature = "serde")]
 /// Functions used by serde impls of all hashes.
+#[cfg(feature = "serde")]
 pub mod serde_details {
     use Error;
 
@@ -119,10 +119,10 @@ pub mod serde_details {
     }
 }
 
-#[macro_export]
-#[cfg(feature = "serde")]
 /// Implements `Serialize` and `Deserialize` for a type `$t` which
 /// represents a newtype over a byte-slice over length `$len`.
+#[macro_export]
+#[cfg(feature = "serde")]
 macro_rules! serde_impl(
     ($t:ident, $len:expr) => (
         impl $crate::serde_macros::serde_details::SerdeHash for $t {