message: Add a note about how CallbackContext can be copied by code outside `libtock_platform`.
diff: @@ -63,8 +63,10 @@ pub trait MethodCallback<AsyncResponse> {
  pub struct CallbackContext<'c> {
  // `_phantom` serves three purposes. It uses the `c lifetime to avoid an
  // "unused lifetime" error, it provides the proper variance over 'c, and it
  - // prevents code outside this crate from constructing a CallbackContext
  - // (because of its visibility control).
  + // prevents code outside this crate from directly constructing a
  + // CallbackContext (because of its visibility control). Code outside this
  + // crate can copy a CallbackContext, but that is fine as copying the
  + // CallbackContext preserves its associated lifetime.
  pub(crate) _phantom: core::marker::PhantomData<&'c ()>,
  }
  