Update system-variables.md

[joey-yez]

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• This PR is translated from:Update system-variables.md docs-cn#16092
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[windtalker]

LGTM

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[dveeden]

It seems that these variables are mentioned in transaction-isolation-levels.md. Should we add a link?

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-10 08:11:39.539080256 +0000 UTC m=+430889.123333946: ☑️ agreed by ran-huang.

[ran-huang]

/approve

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[ran-huang]

It seems that these variables are mentioned in transaction-isolation-levels.md. Should we add a link?

@dveeden Added. Thank you!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #16075.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #16076.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.3: #16077.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.4: #16078.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #16079.

[ti-chi-bot]

In response to a cherrypick label: cannot checkout release-7.6: error checking out release-7.6: exit status 1. output: error: pathspec 'release-7.6' did not match any file(s) known to git

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #16080.