Improve documentation for split function.
Improve documentation for split function.

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