"Undocumented" warnings should be fixed, but that can wait.
However, all other warnings, like @ingroup without corresponding @defgroup, unknown doxygen commands (like \prief) and many others should be fixed.
Additionally, make doc should fail if a doxygen error other than "undocumented" is found.
See D7126#123795 for an example problem.
This relates to T5833 but for all doxygen documentation, not only Legacy.