[DEV][DOC] Add some hints to the mysql documentation

[ic4y]

Purpose of this pull request

Check list

Add some hints to the mysql documentation

• [x] Code changed are covered with tests, or it does not need tests for reason:
• [x] If any new Jar binary package adding in your PR, please add License Notice according New License Guide
• [x] If necessary, please update the documentation to describe the new feature. https://github.com/apache/incubator-seatunnel/tree/dev/docs