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

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

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: 683a63befc73 ("rust: platform: add basic platform device / driver abstractions")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250324210359.1199574-9-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>