move doc(hidden)

i forgot that `#![doc(hidden)]` applies to the module, not everything in
it... again. (this caused linkchecker to fail, because of a debug impl)

Author: WaffleLapkin

library/core/src/ops/index_range.rs

@@ -1,5 +1,4 @@
 #![unstable(feature = "std_internals", issue = "none")]
-#![doc(hidden)]
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::num::NonZero;
 use crate::ub_checks;
@@ -11,6 +10,7 @@ use crate::ub_checks;
 /// (Normal `Range` code needs to handle degenerate ranges like `10..0`,
 ///  which takes extra checks compared to only handling the canonical form.)
 #[derive(Clone, Debug, PartialEq, Eq)]
+#[doc(hidden)]
 pub struct IndexRange {
     start: usize,
     end: usize,