docs/devices.md updates for battery and HID++ protocol version

HID++ protocol version is quite important for feature availability, therefore I decided to document that.

I have also mentioned that the K800 keyboard is rechargeable over USB (no separate batteries). Should this be documented separately as I did? All Unifying devices have batteries (they are wireless...). The HID++ protocol is not limited to USB, so it should also work for Bluetooth devices (support in later versions?). If there are no intents to support those, what is the point of having a battery column?

Perhaps you can document the headers on top of the tables like I did.

Thanks,
Peter