rust: enable `clippy::unnecessary_safety_doc` lint

In Rust 1.67.0, Clippy added the `unnecessary_safety_doc` lint [1],
which is similar to `unnecessary_safety_comment`, but for `# Safety`
sections, i.e. safety preconditions in the documentation.

This is something that should not happen with our coding guidelines in
mind. Thus enable the lint to have it machine-checked.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#/unnecessary_safety_doc

 [1]
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/20240904204347.168520-7-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>