[geometry] Fixes for Doxygen: we go back to ifdef DOXYGEN... for parameters which should not be documented, instead of qbk_skip. This because the newer versions of Doxygen did not support the qbk_skip construction anymore.
Also removed <em> from the comment because it complained about the end (there was a /2, meaning divide by two in between)

svn path=/trunk/boost/geometry/; revision=83714