system-variables: improve noop functions warning (#6374)

[ti-chi-bot]

This is an automated cherry-pick of #6374

What is changed, added or deleted? (Required)

The feature 'noop functions' is actually very unsafe. Several internal users I have spoken to recently have misunderstood it.

This changes it from a 'note' to a 'warning' and provides an example for more context of what it does.

The example will need to be changed slightly for TiDB 5.1 and below because transaction read only is not affected by noop funcs yet.

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For details, see tips for choosing the affected versions.

• master (the latest development version)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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• Delete files
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4a6fdb7