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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@pepezzzz This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

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@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

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

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