Update some stale comments

[k-sareen]

No description provided.

[wks]

LGTM, but object_reference_write_slow is a method of trait Barrier, but Barrier is private, and there is no wrapper for it in memory_manager.rs. I wonder why the OpenJDK binding is able to call it via

        mutator
            .barrier()
            .object_reference_write_slow(src, slot.into(), target);

[k-sareen]

Weird. I'm not sure. There's no pub use barriers::Barrier anywhere.