DocDataTextWriter::Flush Method ()

Keeps track of how much we have sent to the text stream and clears the string builder effectively implementing the expected behavior of flushing a text stream. This method pushes data into the stream only if there is data to push. If the contents of the writer have not been changed, this method will not delete the data in the text stream. It is implemented this way so that client service providers may utilize more advanced flushing schemes.