Doxygen: how to make sure internal features are not exported to OnlineHelp and PDFs

0. doxy-config file (most of the times names .doxy) should contain
:INTERNAL_DOCS = NO


1. Doxygen-blocks for classes should be wrapped with

/// @cond INTERNAL
/// @ingroup whateverGRP
///

/// @endcond

2. Maybe use the keyword ‘internal’ for its real purpose 😉

Additional note: only use
for newlines in Doxygen in C#. Else prepare for:
Severity Code Description Project File Line Suppression State
Warning CS1570 XML comment has badly formed XML -- 'Expected an end tag for element 'myTag'.'"