I really, really hope to write an actual manual from scratch some
day
Have you considered using Doxygen http://www.doxygen.org/ to
embedded the documentation right in the software? Makes keeping
both up to date easier.
For the type of manual we're talking about, source comments are
absolutely useless as the basis for it. They do not have the proper
structure to correspond to a user's guide.
At best, source comments can generate useful API manuals, and
references for the elements of the software.
It's still a valid suggestion.