rust: error: extend the Result documentation

Extend the Result documentation by some guidelines and examples how
to handle Result error cases gracefully. And how to not handle them.

While at it fix one missing `Result` link in the existing documentation.

[ Moved links out-of-line for improved readability. Fixed `srctree`
  link. Sorted out-of-line links. Added newlines for consistency
  with other docs. Applied paragraph break suggestion. Reworded
  slightly the docs in a couple places. Added Markdown.

  In addition, added `#[allow(clippy::single_match)` for the first
  example. It cannot be an `expect` since due to a difference introduced
  in Rust 1.85.0 when there are comments in the arms of the `match`.
  Reported it upstream, but it was intended:

      https://github.com/rust-lang/rust-clippy/issues/14418

  Perhaps Clippy will lint about it in the future, but without autofix:

      https://github.com/rust-lang/rust-clippy/pull/14420

    - Miguel ]

Link: https://lore.kernel.org/rust-for-linux/CANiq72keOdXy0LFKk9SzYWwSjiD710v=hQO4xi+5E4xNALa6cA@mail.gmail.com/


Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20250122054719.595878-1-dirk.behme@de.bosch.com


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