xso-proc: hide generated FromXml / AsXml types

They really clutter the documentation, because they show up in the
module's index. Downside is that users can't inspect the documentation
of those types anymore, but it's not useful anyway: the parts are
private and they just impl some traits.

xso-proc/src/state.rs

@@ -520,6 +520,7 @@ impl FromEventsStateMachine {
             }
 
             #[doc = #docstr]
+            #[doc(hidden)]
             #vis struct #builder_ty_ident(::core::option::Option<#state_ty_ident>);
 
             impl ::xso::FromEventsBuilder for #builder_ty_ident {
@@ -698,6 +699,7 @@ impl AsItemsStateMachine {
             }
 
             #[doc = #docstr]
+            #[doc(hidden)]
             #vis struct #item_iter_ty(::core::option::Option<#state_ty_ident<#item_iter_ty_lifetime>>);
 
             impl<#item_iter_ty_lifetime> ::core::iter::Iterator for #item_iter_ty {