Add some explanation for tiflash tuning performance doc

[yibin87]

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[xzhangxian1008]

LGTM

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[LGTM Timeline notifier]

Timeline:

• 2023-11-24 08:05:47.492696563 +0000 UTC m=+564376.157922758: ☑️ agreed by qiancai.

[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #15498.

[CrystalAnalyst]

/assign

[qiancai]

/assign @CrystalAnalyst