system-variables: improve noop functions warning

[en-jin19]

What is changed, added or deleted? (Required)

Update the "warning" part of tidb_enable_noop_functions in "system-variables.md".

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• 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)

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

• This PR is translated from: system-variables: improve noop functions warning docs#6374
• 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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

LGTM

[TomShawn]

/merge

[ti-chi-bot]

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

Details

Commit hash: 5ef9e8d

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #7136.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #7137.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #7138.