Do you have Access or Visio with your level of Office?
I'm a consultant mostly doing interfaces for Hospitals and I have an Excel workbook I use to document them. I usually implement a high-level diagram when there are multiple interfaces involved. I use the "Quick Shapes" and hyperlink them to either another spreadsheet file, or somewhere within the workbook.
You can get fancier with Access or Visio, though.
I am fiddling around with an Access-based application to do that, but am not on a schedule or anything....