Add comment to explain the IPV6_EX suffix. The confusion about these RSS hash type probably stems from the facts that they were never widely implemented by hardwares.
This seems generally sensible.
I'm not quite sure about keeping the "Non-standard RSS hash types" comment, as M_HASHTYPE_OPAQUE is basically intended to be a catch-all for any other model, and is not specifically intended to be RSS (hence not having RSS in its name). So I'd be tempted to drop that comment.