add more info in tiflash 620 upgrade guide

[lidezhu]

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[ti-chi-bot]

[REVIEW NOTIFICATION]

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• shichun-0415

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[shichun-0415]

/remove status LGT1
/status LGT2
/merge

[ti-chi-bot]

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

Commit hash: fd62b69