Add HEKA digital port information to docs

[cafarm]

Per Sam Cooler:

I spent a while dealing with an issue today, getting digital output to work from the ITC-18. Turns out, the ITC front panel is doport1, not doport0, and that the bit position is what is equivalent to Symphony 1's DIGITAL_OUT.x label.

Though I've got it figured out now, I think a mapping from physical ports to port name strings (and bit position) would be helpful for others, for the future.

Ah, I meant a text information mapping. I'm just saying to add to the symphony documentation, or example rig configs, or somewhere, the words "The ITC-18 front panel digital lines are doport1, and are accessed individually using the bit positions"