make document better

[amyangfei]

Feature Request

Is your feature request related to a problem? Please describe:

We are constantly working on improving the DM document, to achieve better accuracy, completeness and readability. We will list some TODO task in the following and feel free to complete them.

Describe the feature you'd like:

• [x] document about online-ddl usage and configuration
• [x] continuous improve the trouble shooting part
• [x] add ignore-checking-items configuration description
• [x] add document about privilege for the heartbeat feature
• [ ] add document about checkpoint feature
• [ ] ~notice user change relay-related config when upgrade from 1.0~

[csuzhangxc]

Does ignore-checking-items need to be documented?

[amyangfei]

Does ignore-checking-items need to be documented?

yep, added in list

[lance6716]

seems privilege for heartbeat feature already here: https://docs.pingcap.com/zh/tidb-data-migration/stable/feature-overview#%E7%B3%BB%E7%BB%9F%E6%9D%83%E9%99%90