Add Comments Explaining RCI AI/AO

[ZLLentz]

Our twincat general library has function blocks for handling analog input and output: https://github.com/pcdshub/lcls-twincat-general/blob/master/LCLSGeneral/LCLSGeneral/POUs/Hardware/FB_AnalogInput.TcPOU

But these were not used in #6.
We should add some comments/documention as to why they needed to be modified, and/or we should add an issue to the general lib to fix these FBs.

[slactjohnson]

I have opened up an issue at pcdshub/lcls-twincat-general#54. The tl;dr is that these failed for negative values and I had to make some quick changes.