rust: sync: Clean up LockClassKey and its docs

rust: sync: Clean up LockClassKey and its docs

Several aspects of the code and documentation for this type is
incomplete. Also several things are hidden from the docs. Thus, clean it
up and make it easier to read the rendered html docs.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20250811-lock-class-key-cleanup-v3-2-b12967ee1ca2@google.com