Add doc(hidden) to all __iterator_get_unchecked

This method on the Iterator trait is doc(hidden), and about half of
implementations were doc(hidden). This adds the attribute to the
remaining implementations.

library/core/src/array/iter.rs

@@ -132,6 +132,7 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
     }
 
     #[inline]
+    #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
     where
         Self: TrustedRandomAccess,