Curve comment looks misleading

Hi,

I'm not sure if this is a real issue but it tripped me up:

https://gitlab.com/sequoia-pgp/sequoia/-/blob/main/openpgp/src/types/mod.rs#L309-312

/// Note: This enum cannot be exhaustively matched to allow future
/// extensions.
#[derive(Debug, Clone, Hash, PartialEq, Eq, PartialOrd, Ord)]
pub enum Curve {

The comment says that the enum cannot be exhaustively matched but the enum is not marked with #[non_exhaustive] just like others: https://gitlab.com/sequoia-pgp/sequoia/-/blob/main/openpgp/src/types/mod.rs#L98

Is this an issue? There's an Unknown variant but I don't know if that's what the commenter meant.