Add documentation to parseMathML.
Add documentation to parseMathML.

libcellml/buildbot-testing,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,hsorby/libcellml,libcellml/buildbot-testing,cellml/libcellml,MichaelClerx/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,nickerso/libcellml,cellml/libcellml