dumpling: clarify description about --csv-separator (#21126)

[ti-chi-bot]

This is an automated cherry-pick of #21126

--csv-separator $(echo -e "\x01")

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add --csv-separator $(echo -e "\x01") commets

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Test name	Commit	Details	Required	Rerun command
pull-verify	0777dd6	link	true	/test pull-verify

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