Make it clearer that the MathML syntax errors are LibXml2 specific.
Make it clearer that the MathML syntax errors are LibXml2 specific.

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