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 do

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 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 those two instances.

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: https://github.com/rust-lang/rust/pull/129531 [1]
Link: https://github.com/rust-lang/rust-clippy/pull/12993 [2]
Link: https://rust-lang.github.io/rust-clippy/master/index.html#/too_long_first_doc_paragraph [3]
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-15-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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