rust: provide proper code documentation titles

Rust 1.82.0's Clippy is introducing [1][2] a new warn-by-default lint,
`too_long_first_doc_paragraph` [3], which is intended to catch titles
of code documentation items that are too long (likely because no title
was provided and the item documentation starts with a paragraph).

This lint does not currently trigger anywhere, but it does detect a couple
cases we had in private cases if checking for private items gets enabled
(which we will do in the next commit):

    error: first doc comment paragraph is too long
      --> rust/kernel/init/__internal.rs:18:1
       |
    18 | / /// This is the module-internal type implementing `PinInit` and `Init`. It is unsafe to create this
    19 | | /// type, since the closure needs to fulfill the same safety requirement as the
    20 | | /// `__pinned_init`/`__init` functions.
       | |_
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#too_long_first_doc_paragraph
       = note: `-D clippy::too-long-first-doc-paragraph` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(clippy::too_long_first_doc_paragraph)]`

    error: first doc comment paragraph is too long
     --> rust/kernel/sync/arc/std_vendor.rs:3:1
      |
    3 | / //! The contents of this file come from the Rust standard library, hosted in
    4 | | //! the <https://github.com/rust-lang/rust> repository, licensed under
    5 | | //! "Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,
    6 | | //! see <https://github.com/rust-lang/rust/blob/master/COPYRIGHT>.
      | |_
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#too_long_first_doc_paragraph

Thus clean the two instances we hit and enable the lint.

In addition, since we have a second `std_vendor.rs` file with a similar
header, do the same there too (even if that one does not trigger the lint,
because it is `doc(hidden)`).

Link: rust-lang/rust#129531 [1]
Link: rust-lang/rust-clippy#12993 [2]
Link: https://rust-lang.github.io/rust-clippy/master/index.html#/too_long_first_doc_paragraph [3]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/__internal.rs

@@ -15,9 +15,10 @@ use super::*;
 /// [this table]: https://doc.rust-lang.org/nomicon/phantom-data.html#table-of-phantomdata-patterns
 pub(super) type Invariant<T> = PhantomData<fn(*mut T) -> *mut T>;
 
-/// This is the module-internal type implementing `PinInit` and `Init`. It is unsafe to create this
-/// type, since the closure needs to fulfill the same safety requirement as the
-/// `__pinned_init`/`__init` functions.
+/// Module-internal type implementing `PinInit` and `Init`.
+///
+/// It is unsafe to create this type, since the closure needs to fulfill the same safety
+/// requirement as the `__pinned_init`/`__init` functions.
 pub(crate) struct InitClosure<F, T: ?Sized, E>(pub(crate) F, pub(crate) Invariant<(E, T)>);
 
 // SAFETY: While constructing the `InitClosure`, the user promised that it upholds the

rust/kernel/std_vendor.rs

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
+//! Rust standard library vendored code.
+//!
 //! The contents of this file come from the Rust standard library, hosted in
 //! the <https://github.com/rust-lang/rust> repository, licensed under
 //! "Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,

rust/kernel/sync/arc/std_vendor.rs

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
+//! Rust standard library vendored code.
+//!
 //! The contents of this file come from the Rust standard library, hosted in
 //! the <https://github.com/rust-lang/rust> repository, licensed under
 //! "Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,