STYLE: update Markups doxygen documentation

Moved sections so that they'll be visible on the generated pages.
Expanded documentation on the node level versus markup level settings.
Added separate comments for each documented method.



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22481 3bd1e089-480b-0410-8dfb-8563597acbee