rust: init: update expanded macro explanation

rust: init: update expanded macro explanation

The previous patches changed the internals of the macros resulting in
the example expanded code being outdated. This patch updates the example
and only changes documentation.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-14-benno.lossin@proton.me
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>