Docs: Update for Doxygen 1.8.9
Docs: Update for Doxygen 1.8.9

* docs/reference/Doxyfile.in: Update for Doxygen 1.8.9.

boomuuh/libxml-cpp,dtrebbien/libxmlmm,GNOME/libxmlmm,xf22001/libxmlpp,GNOME/libxmlmm,GNOME/libxmlmm,boomuuh/libxml-cpp,xf22001/libxmlpp,boomuuh/libxml-cpp,xf22001/libxmlpp,dtrebbien/libxmlmm,dtrebbien/libxmlmm,GNOME/libxmlmm