doc: fix obsolete tags warnings

When Doxygen generates the documentation (via `make doxygen-run`), it displays warnings such as:
warning: Tag 'OUTPUT_TEXT_DIRECTION' at line 102 of file 'Doxyfile' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

The patch contains only the modifications done by `doxygen -u`.

Author: sblondon