docs:按照文档部署后，看到监听的端口有点懵，所以补充了监听端口用途描述

[geekgao]

Description

Fixes # (issue)

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [ ] I have added tests corresponding to this change
• [ ] I have updated the documentation to reflect this change
• [ ] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)