When I had to make documentation files to my last project in C# I had to use sandcastle which required creating a new project in my solution just for documentation. It was quite a complicated process and took me some time to understand why I had to all these steps in the first place. Back then I wondered if there exists an easier way - I just wanted to give a c# file and get a generated doc file.
Luckily there is an easier way thanks to SHFB (Sandcastle Help File Builder). You just point a solution and SHFB generates documentation for you.