rust: slice: fix broken intra-doc links

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: 88622323dde3 ("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>