Starting doxygenising source code

[Jordi Gutiérrez Hermoso]

I have been sitting for over a year on the following patch that I just
pushed:

    http://hg.savannah.gnu.org/hgweb/octave/rev/96751a74bbbb

The intent here is to just get the ball rolling on writing proper
Doxygen comments for our code. I have opted to go with a style similar
to what we do with texinfo, i.e. @ instead of \ for doing Doxygen
commands. Short one-line comments can be done \\!. These are the
relevant instructions on the Doxygen markup language:

    http://www.stack.nl/~dimitri/doxygen/manual/docblocks.html

Hopefully we can start to make some sense out of the chaos.

- Jordi G. H.