rust: block: fix formatting of `kernel::block::mq::request` module

Fix several issues with rustdoc formatting for the
`kernel::block::mq::Request` module, in particular:

  - An ordered list not rendering correctly, fixed by using numbers
    prefixes instead of letters.

  - Code snippets formatted as regular text, fixed by wrapping the
    code with `back-ticks`.

  - References to types missing intra-doc links, fixed by wrapping the
    types with [square brackets].

Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://github.com/Rust-for-Linux/linux/issues/1108


Signed-off-by: Francesco Zardi <frazar00@gmail.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Fixes: 3253aba3 ("rust: block: introduce `kernel::block::mq` module")
Link: https://lore.kernel.org/r/20240903173027.16732-3-frazar00@gmail.com


[ Added an extra intra-doc link. Took the chance to add some periods
  for consistency. Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>