ticdc: add more docs about unified sorter (#6258)

[ti-chi-bot]

This is an automated cherry-pick of #6258

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[TomShawn]

@overvenus Please help update this sentence for release-4.0. Thanks~

[ti-chi-bot]

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[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 30222fa