[devops] fix ci for document check

[ver217]

Overview

https://github.com/hpcaitech/ColossalAI/actions/runs/4977382383/jobs/8906746190?pr=3729#step:5:14

This shows that it failed to update the main branch when merging. We expect that the main branch should be the merged (latest) branch. But now, the main branch is origin main branch.

We can directly checkout main branch from curBranch, so that the main branch is definitely the merged (latest) branch.

Goal

1. Fix branch merge behavior.
2. Update comments.