dumpling: update privilege related section

[joechenrh]

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What is changed, added or deleted? (Required)

The privilege section of dumpling is misleading.

• First, MySQL also has RELOAD privilege.
• Second, --consistency flush can't apply to TiDB.

https://github.com/pingcap/tidb/blob/23de08cfb549df008edd69f3a9a7313ab18ecff3/dumpling/export/consistency.go#L93-L98

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• master (the latest development version)
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[LGTM Timeline notifier]

Timeline:

• 2025-06-05 02:23:34.099501346 +0000 UTC m=+494900.301846611: ☑️ agreed by tangenta.
• 2025-06-05 02:30:11.782859069 +0000 UTC m=+495297.985204328: ☑️ agreed by hfxsd.

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20467.