rust: types: add intra-doc links for `Opaque<T>`

We use intra-doc links wherever possible. Thus add a couple missing ones
for `Opaque<T>`.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250305053438.1532397-1-dirk.behme@de.bosch.com


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