system-variables: update docs for hint SET_VAR

[shawn0915]

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[Oreoxmt]

/cc @winoros

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-10-25 02:22:12.028969529 +0000 UTC m=+2401329.616079667: ☑️ agreed by hfxsd.
• 2023-10-31 01:59:03.204742256 +0000 UTC m=+2918340.791852401: ☑️ agreed by Oreoxmt.

[Oreoxmt]

/approve

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.4: #15334.