Add some explanation for tiflash tuning performance doc (#16045)

[ti-chi-bot]

This is an automated cherry-pick of #16045

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What is changed, added or deleted? (Required)

Add some explanation for tiflash tuning performance doc

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• v7.6 (TiDB 7.6 versions)
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What is the related PR or file link(s)?

• This PR is translated from: Add some explanation for tiflash tuning performance doc docs-cn#15496
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Oreoxmt]

/approve

[ti-chi-bot]

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