rust: slice: fix broken intra-doc links

In older versions of Rust, the compiler doesn't know about the newer
`as_flattened*` methods, thus `rustdoc` complains about the intra-doc
links, e.g.

     error: unresolved link to `slice::as_flattened`
      --> rust/kernel/slice.rs:19:23
       |
    19 | /// [`as_flattened`]: slice::as_flattened
       |                       ^^^^^^^^^^^^^^^^^^^ the primitive type `slice` has no associated item named `as_flattened`
       |
       = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

Thus fix them by using an URL instead.

Fixes: 8862232 ("rust: enable slice_flatten feature and provide it through an extension trait")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251119185125.1411151-1-ojeda@kernel.org
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: ojeda

rust/kernel/slice.rs

@@ -16,22 +16,22 @@
 ///
 /// This trait can be removed once the MSRV passes 1.80.
 ///
-/// [`as_flattened`]: slice::as_flattened
-/// [`as_flattened_mut`]: slice::as_flattened_mut
+/// [`as_flattened`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_flattened
+/// [`as_flattened_mut`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_flattened_mut
 #[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
 pub trait AsFlattened<T> {
     /// Takes a `&[[T; N]]` and flattens it to a `&[T]`.
     ///
     /// This is an portable layer on top of [`as_flattened`]; see its documentation for details.
     ///
-    /// [`as_flattened`]: slice::as_flattened
+    /// [`as_flattened`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_flattened
     fn as_flattened(&self) -> &[T];
 
     /// Takes a `&mut [[T; N]]` and flattens it to a `&mut [T]`.
     ///
     /// This is an portable layer on top of [`as_flattened_mut`]; see its documentation for details.
     ///
-    /// [`as_flattened_mut`]: slice::as_flattened_mut
+    /// [`as_flattened_mut`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_flattened_mut
     fn as_flattened_mut(&mut self) -> &mut [T];
 }