devantech-eth008: add more supported models, variable DI width, doc cleanup

Add support for differing DI (digital input) response sizes depending
on the model. Declare support for more models, including WIFI (their
feature set and protocol are identical, only their model ID differs).

Rework developer notes, keep the API slim and concentrate most stuff
in the implementation. Discuss USB CDC protocols and other potential
TODO items. Modbus most probably remains out of that driver's scope.

This implementation should cover most Ethernet (and Wifi) models, and
support their complete feature set from the sigrok perspective. The
most important limitation is missing support for channels' interaction
(when a pin can be any of AI and DI and DO).