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 <[email protected]>
Suggested-by: Miguel Ojeda <[email protected]>
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 <[email protected]>
Signed-off-by: Benno Lossin <[email protected]>
Signed-off-by: Dirk Behme <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ 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 <[email protected]>

0 files changed, 0 insertions, 0 deletions