Fix doxygen documentation in internaltypes.h.
Fix doxygen documentation in internaltypes.h.

cellml/libcellml,nickerso/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml