API dox: move @cond/@endcond to doxygen comment style, to unconfuse doxygen
Doxygen currently does not generate documentation for e.g. Akonadi::Item, as it seems to fail to properly parse the code (not sure exactly, might be the template methods) and starts to misinterpret things. Then all the conditional section markers are put into non-doxygen comments so at least recent doxygen versions ignore them (given most markers have been added around a decade ago perhaps earlier doxygen processed them). But with doxygen getting confused while parsing, it seems to suddenly handle some of the @cond and €endcond markers, but missing out the matching start @cond or ending €endcond, and with that fail even harder to parse the rest.
Moving the markers into doxygen-processed comment style improves the doxygen handling, and e.g. Akonadi::Item now gets documented