What is the best way to prove to my boss that in code documentation is greater than extensive external documents containing documentation and screenshots of code/ui?

We have a group in the company running SAP code. Their documentation process involves hours of laborious documentation and screenshots of the code and the interface (before and after).

They want us (C# developers) to follow the same rubric as they do. I want to argue that using comments in the code, and a great, self documenting language such as C# is fulfilling the need for documentation the same as, or greater than the other team's document method.

Is there any published research by big companies that say external documentation is out dated and that in code comments are more efficient? (or something similar to this point)

Edit - This has nothing to do with users. this is NOT user training documentation. Only internal use