rust: pin-init: use Markdown autolinks in Rust comments

"Normal" comments in Rust (`//`) are also formatted in Markdown, like
the documentation (`///` and `//!`), see
Documentation/rust/coding-guidelines.rst

Thus use Markdown autolinks for a couple links that were missing it.

It also helps to get proper linking in some software like kitty [1].

Suggested-by: Benno Lossin <benno.lossin@proton.me>
Link: https://github.com/Rust-for-Linux/pin-init/pull/32#discussion_r2023103712

 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/pin-init/pull/32/commits/dd230d61bf0538281072fbff4bb71efc58f3420c


Fixes: 84837cf6 ("rust: pin-init: change examples to the user-space version")
Cc: stable@vger.kernel.org
[ Change case in title. Reworded commit message. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250407201755.649153-3-benno.lossin@proton.me


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