rust: task: add missing Markdown code spans and intra-doc links

Add missing Markdown code spans and also convert them into intra-doc
links.

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

Fixes: e0020ba6 ("rust: add PidNamespace")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250324210359.1199574-10-ojeda@kernel.org


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