To reiterate what John has stated, the best way is to write your XML comments in code while you write your application code. Next, you can use a documentation generator, like
Sandcastle to build multiple different kinds of documentation.
Also, under VS project options: Build->Output->(check) XML Documentation file
This will write an XML documentation file when you Build your code from your comments. The XML can then be used by other tools, such as Sandcastle, to build rich documentation for you client.