What is Doxygen ?
Doxygen is the de facto standard tool for generating documentation from annotated C++ sources 
Generating the Doxygen documentation for Octave
Doxygen documentation for Octave is easily generated from Octave sources. Instructions can be found in doc/doxyhtml/README.
Where can I browse the Doxygen documentation for Octave ?
Alternatively, you can access version specific Doxygen documentation:
What is the current status of Octave's Doxygen documentation ?
Very little Doxygen documentation is currently available in Octave's source code. Which is why "Doxygen documentation for the C++ classes" is listed in the Projects page of this wiki.
Doxygen Style Guide
For the Doxygen Style Guide look at C++_style_guide#Doxygen.