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 to 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.
2 jobs for !50 with work/kossebau/helpdoxygen in 17 minutes and 29 seconds (queued for 118 minutes and 21 seconds)
latest detached
Status Job ID Name Coverage
  Build
failed #94684
build_clazy_clang_tidy

00:04:38

passed #94683
linux kf5-qt5 qt5.15

00:12:51

1.0%
 
Name Stage Failure
failed
build_clazy_clang_tidy Build
make[2]: *** [src/shared/CMakeFiles/akonadi_shared.dir/build.make:121: src/shared/CMakeFiles/akonadi_shared.dir/akremotelog.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:2627: src/shared/CMakeFiles/akonadi_shared.dir/all] Error 2
make: *** [Makefile:160: all] Error 2
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WARNING: build/clang-tidy-report.xml: no matching files
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Cleaning up file based variables
ERROR: Job failed: exit code 1