Minor improvement to Basal's documentation (in the right place)

[giacecco]

@modlfo, thank you so much for your amazing work with Vult. Hope you don't mind if I suggest this little addition to the documentation.

I added information about the voltage represented by the Mod 1 and 2 encoders and how the CV input and related attenuverters behave. Hope I got it right from my experiments 😀