Update docs for DAC Expansion Board

[codecubepi]

This update should include two things:

1. Documentation of the board re-work needed on REV B and REV C DAC boards' U9 IC (see this issue)
2. Updated documentation on using the latest DAC Board revision. Ideally, a new driver will be created that will support using the DAC REV C and REV D via the GPIO Port on AMDC REV E and beyond.