// seves document to file starting from the root-node of
// the document (provided by GetTopSection() method),
// or from "pFromSection" if it's not NULL.
// fopenOptions arg. is string passed to fopen() method,
// seves document to file starting from the root-node of
// the document (provided by GetTopSection() method),
// or from "pFromSection" if it's not NULL.
// fopenOptions arg. is string passed to fopen() method,