v7.5.1: tikv configuration doc: add gc thread number (#16564)

[ti-chi-bot]

This is an automated cherry-pick of #16564

What is changed, added or deleted? (Required)

v7.5.1 adds num-threads

Which TiDB version(s) do your changes apply to? (Required)

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• master (the latest development version)
• v8.0 (TiDB 8.0 versions)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
• v7.4 (TiDB 7.4 versions)
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• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

What is the related PR or file link(s)?

• This PR is translated from: v7.5.1: tikv configuration doc: add gc thread number docs-cn#16628
• 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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