hashes: Hide both macros

We have two macro definitions feature gated on `serde`. At some stage we
added the `doc(hidden)` attribute to one of them but forgot to add it to
the other. This technically makes our features non-additive. This macro
is "internal" so its unlikely that this is being used in the wild.

Add `doc(hidden)` to the `serde_impl` macro that is missing it.

Found by `cargo semver-checks` after recent upgrade to 0.38

hashes/src/macros.rs

@@ -533,6 +533,7 @@ macro_rules! serde_impl(
 ));
 
 /// Does an "empty" serde implementation for the configuration without serde feature.
+#[doc(hidden)]
 #[macro_export]
 #[cfg(not(feature = "serde"))]
 macro_rules! serde_impl(