C++ source code is highly intricate and detail-specific, Doxygen documentation is very simple and big picture-oriented, these are the differences and advantages/disadvantages in a nutshell.
I've changed the otclient framework to load all the modules/data embedded and ignore the filesystem/physfs, I'm by no means an expert, but I know at least enough to have done that, but especially in the beginning when I knew absolutely nothing about otclient, doxygen documentation would've helped me out a lot since a lot of my time was spent looking for function calls manually using grep for days/weeks.
Not to mention the source code is available as well through the web interface.
Feel free not to use it, either way it answers OP's question about otclient documentation.