rust: pci: fix docs related to missing Markdown code spans

rust: pci: fix docs related to missing Markdown code spans

In particular:

  - Add missing Markdown code spans.

  - Improve title for `DeviceId`, adding a link to the struct in the
    C side, rather than referring to `bindings::`.

  - Convert `TODO` from documentation to a normal comment, and put code
    in block.

This was found using the Clippy `doc_markdown` lint, which we may want
to enable.

Fixes: 1bd8b6b2c5d3 ("rust: pci: add basic PCI device / driver abstractions")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250324210359.1199574-8-ojeda@kernel.org
[ Prefixed link text with `struct`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>