Document passing a Hash parameter to on_duplicate_key_update

I needed to use custom values in a `multi_insert` with `on_duplicate_key_update`, and I couldn't figure it out or find it in the documentation. I ended up finding it [here](https://groups.google.com/forum/#!searchin/sequel-talk/on$20duplicate$20key/sequel-talk/Ho1Fy79jdrY/9FGaj6wCImAJ).