rust: types: extend `Opaque` documentation

Update the `Opaque` documentation and add an example as proposed by
Miguel Ojeda in [1]. The documentation update is mainly taken from
Benno Lossin's description [2].

Cc: Nell Shamrell-Harrington <nells@linux.microsoft.com>
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://rust-for-linux.zulipchat.com/#narrow/stream/291565/topic/x/near/467478085 [1]
Link: https://rust-for-linux.zulipchat.com/#narrow/stream/291565/topic/x/near/470498289

 [2]
Co-developed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://lore.kernel.org/r/20241002050301.1927545-1-dirk.behme@de.bosch.com


[ Used `expect`. Rewrapped docs. Added intra-doc link. Formatted
  example. Reworded to fix tag typo/order. Fixed `&mut` formatting
  as discussed. Added Benno's SOB and CDB as discussed. Shortened
  links. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>