I don't know about you...but I absolutely dislike drawing out this documentation by hand. So, let me tell you that I was pleasantly surprised to see this Sequence Diagram Generator from Nauman Leghari's blog.

After waiting an interminable amount of time (20 minutes) for the GNU download, I finally got a chance to try it out, and, let me tell you, it's worth the price (free) and more. It doesn't look like it handles cross assembly calls...but that's easily fixable, and the source code is provided.