rust: arc: fix intra-doc link in Arc<T>::init

`Arc<T>::init` refers to `Arc<T>::pin_init` via an intra-doc link
using the text `pin_init`, rather than more explicitly, which makes
`rustdoc` point it to the `pin_init!` macro instead.

This is required for the compiler upgrade since the newer `rustdoc`
would trigger the `broken_intra_doc_links` lint [1], but in this case
the macro was not the intended target to begin with, and so the actual
fix is to make it point to the right place, regardless of the upgrade.

Thus make it more explicit.

Fixes: 92c4a1e ("rust: init/sync: add `InPlaceInit` trait to pin-initialize smart pointers")
Link: rust-lang/rust#106142 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Diff for: rust/kernel/sync/arc.rs

@@ -185,7 +185,7 @@ impl<T> Arc<T> {
 
     /// Use the given initializer to in-place initialize a `T`.
     ///
-    /// This is equivalent to [`pin_init`], since an [`Arc`] is always pinned.
+    /// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
     #[inline]
     pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
     where