I believe the process is to export the report and select Report Definition (txt). Under the File menu -->Export -->Export Report--->Format box
This is a drop down box and select Report Definition (txt). This will create a text file of your report, including formulas, variables, etc. Save the text file to a place you can find it. Then you can open it up in any word processor to make it look nice. Also you could insert some screen shots of the layout to further improve the documentation.
Hope this helps
Rsgards,
John W.