code: public static XWPFRun getRun(int paragraphIndex, int runIndex) {
          createXwpfDocumentIfNull();
          return xwpfDocument.getParagraphs().get(paragraphIndex).getRuns().get(runIndex);
      }
docs: 获取一个XWPFRun对象
  
  @param paragraphIndex XWPFParagraph位置
  @param runIndex XWPFRun位置
  
  @return {@link XWPFRun}