user-defined-variables: Add note about SELECT...INTO <var> (#5268)

[ti-srebot]

cherry-pick #5268 to release-5.0
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# In docs repo:
git pr https://github.com/pingcap/docs/pull/5393

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/docs.git pr/5393:release-5.0-3e1ac172150f

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What is changed, added or deleted? (Required)

user-defined-variables: Add note about SELECT...INTO

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• master (the latest development version)
• 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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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-srebot]

@dveeden you're already a collaborator in bot's repo.

[TomShawn]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

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

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Commit hash: a5b200c