add MySQL compatibility in user-defined-variables.md (#6062)

[ti-srebot]

cherry-pick #6062 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In docs-cn repo:
git pr https://github.com/pingcap/docs-cn/pull/6137

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

git push git@github.com:ti-srebot/docs-cn.git pr/6137:release-5.0-9c1f992124c6

---

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

add a note translation about MySQL compatibility.

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

• 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)

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

• This PR is translated from:user-defined-variables: Add note about SELECT...INTO <var> docs#5268
• 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-srebot]

@Liuxiaozhen12 please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs-cn/invitations

[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.

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Commit hash: 9d83520