std: Mark ptr::Unique with #[doc(hidden)]

Author: Daiki Mizukami

src/libcore/ptr.rs

@@ -2511,6 +2511,7 @@ impl<T: ?Sized> PartialOrd for *mut T {
            reason = "use NonNull instead and consider PhantomData<T> \
                      (if you also use #[may_dangle]), Send, and/or Sync")]
 #[allow(deprecated)]
+#[doc(hidden)]
 pub struct Unique<T: ?Sized> {
     pointer: NonZero<*const T>,
     // NOTE: this marker has no consequences for variance, but is necessary