Complete API documention

[dgollub]

There are still several gaps in API documentation. Doxygen is able to determine where API documentation is wrong/missing if Doxygen is instrumented in the code. This is not always the case. We need full Doxygen coverage as well.

How to generate doxygen documentation

cmake -DBUILD_DOCUMENTATION=true /path/to/opensync/source/
make DoxygenDoc
less doxygen.log

The doxygen logfile doxygen.log gives warnings about wrong/missing documentation. First step would be to instrument each file with the Doxygen base, so we get for each file warnings.