Explain tidb_max_tiflash_threads default value -1 means

[yahonda]

What is changed, added or deleted? (Required)

This pull request explains tidb_max_tiflash_threads default value -1 means in the system-variables.md. The current one just says indicating that this system variable is invalid. Here the invalid is not clear for me.

This commit is based on this line https://github.com/pingcap/docs/blame/e1a67bfe2d66860fa435c8bc851a38451bbd7772/dynamic-config.md#L350

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Do your changes match any of the following descriptions?

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

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-15 06:18:57.240436661 +0000 UTC m=+596228.277663587: ☑️ agreed by Oreoxmt.

[windtalker]

LGTM

[ti-chi-bot]

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[Oreoxmt]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.3: #15796.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.4: #15797.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #15798.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #15799.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #15800.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #15801.

[llkdd1]

/assign