improved doxygen documentation

4 jobs for issue/doxygen_documentation in 33 minutes and 54 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #8601
debian:10 clang-6-libcpp-17

00:31:40

passed #8600
debian:10 gcc-8-17

00:20:58

passed #8602
debian:9 gcc-6-14

00:22:32

passed #8603
ubuntu:18.04 clang-6-17

00:29:22