rust: init: re-enable doctests

Commit a30e94c296 ("rust: init: make doctests compilable/testable")
made these tests buildable among others, but eventually the pin-init
crate was made into its own crate [1] and the tests were marked as
`ignore` in commit 206dea39e5 ("rust: init: disable doctests").

A few other bits got changed in that reorganization, e.g. the
`clippy::missing_safety_doc` was removed and the `expect` use.

Since there is no reason not to build/test them, re-enable them.

In order to do so, tweak a few bits to keep the build clean, and also use
again `expect` since this is one of those places where we can actually
do so.

Link: https://lore.kernel.org/all/20250308110339.2997091-1-benno.lossin@proton.me/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/all/20250526152914.2453949-1-ojeda@kernel.org
Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/kernel/init.rs

@@ -29,15 +29,15 @@
 //!
 //! ## General Examples
 //!
-//! ```rust,ignore
-//! # #![allow(clippy::disallowed_names)]
+//! ```rust
+//! # #![expect(clippy::disallowed_names, clippy::undocumented_unsafe_blocks)]
 //! use kernel::types::Opaque;
 //! use pin_init::pin_init_from_closure;
 //!
 //! // assume we have some `raw_foo` type in C:
 //! #[repr(C)]
 //! struct RawFoo([u8; 16]);
-//! extern {
+//! extern "C" {
 //!     fn init_foo(_: *mut RawFoo);
 //! }
 //!
@@ -66,12 +66,12 @@
 //! });
 //! ```
 //!
-//! ```rust,ignore
-//! # #![allow(unreachable_pub, clippy::disallowed_names)]
+//! ```rust
+//! # #![expect(unreachable_pub, clippy::disallowed_names)]
 //! use kernel::{prelude::*, types::Opaque};
 //! use core::{ptr::addr_of_mut, marker::PhantomPinned, pin::Pin};
 //! # mod bindings {
-//! #     #![allow(non_camel_case_types)]
+//! #     #![expect(non_camel_case_types, clippy::missing_safety_doc)]
 //! #     pub struct foo;
 //! #     pub unsafe fn init_foo(_ptr: *mut foo) {}
 //! #     pub unsafe fn destroy_foo(_ptr: *mut foo) {}