Add doc string for cellmlElementTypeAsString.
Add doc string for cellmlElementTypeAsString.

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