Exception Hunter from RedGate software will get you half-way there. It can do a static analysis of your code and show you what exceptions will be thrown by what lines of code -- including .NET Framework calls. It won't write the XML documentation for you.
But, I have to say, you need to be realistic about how useful such a tool would be... there are a number of exceptions that may happen as a result of extremely unusual circumstances, such as System.OutOfMemoryException and System.ExecutionEngineException, or as a result of programmer error, such as System.NotImplementedException. Technically, these are all possible, but realistically, attempting to document most of them for every method is not worth your time... you'd end up with hundreds of lines of comment for practically every method in your application.
