Add a Caption To an Existing Slide

## Can you think of a better caption?
Each slide can have more than one caption. Check out the current slide show and add a caption (or two).
If you need help, check out the Appendix in the training manual.

查看当前幻灯片放映以选择一个现有幻灯片，如需帮助，请参阅培训手册中的附录。向该幻灯片添加一到两条说明文字；当说明文字在幻灯片放映中可见且内容读起来通顺时，即表示完成。

由索引模型根据 Issue 内容生成。