sysvar: add a note about tidb_service_scope

[hfxsd]

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What is changed, added or deleted? (Required)

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

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• master (the latest development version)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
• v7.4 (TiDB 7.4 versions)
• v7.3 (TiDB 7.3 versions)
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What is the related PR or file link(s)?

• This PR is translated from:sysvar: add a note about tidb_service_scope docs-cn#16397
• Other reference link(s):

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

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@D3Hunter: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-29 08:21:00.938365841 +0000 UTC m=+1380902.502663546: ☑️ agreed by qiancai.

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #16357.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #16358.