add document for DDL

[wjhuang2016]

Signed-off-by: wjhuang2016 huangwenjun1997@gmail.com

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn
• hfxsd

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[hfxsd]

/merge

[ti-chi-bot]

@hfxsd: /merge in this pull request requires 2 approval(s).

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/merge

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[hfxsd]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: f6d0407

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #13247.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #13248.