[doxygen] remove all sources of warnings, if easily possible

Changes only affect documentation, should be possible to merge once todos are removed. @DennisGlaeser : I introduced 3-4 todos when I encountered variables that I don't know the purpose of. Could you clarify this or if you have no time point me to someone who can?

Note: This does not solve all problems with doxygen. I'm going to open an issue to ask for help with the rest of the warnings.