message: Remove AValueHeader from public API
  Summary: Some of the previous diffs made it no longer needed to be doc-hidden.
diff: @@ -47,9 +47,8 @@ pub(crate) struct Arena {
  drop: Bump,
  }
  -#[doc(hidden)] // Appears in a trait, but don't want it to
  #[repr(transparent)]
  -pub struct AValueHeader(DynMetadata<dyn AValue<'static>>);
  +pub(crate) struct AValueHeader(DynMetadata<dyn AValue<'static>>);
  /// How object is represented in arena.
  #[repr(C)]
  