Update the introduction to UCASE() and UNHEX()

[lilin90]

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What is changed, added or deleted? (Required)

Update the introduction of UCASE() and UNHEX(), fix pingcap/docs-cn#15952

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

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• master (the latest development version)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
• v7.4 (TiDB 7.4 versions)
• v7.3 (TiDB 7.3 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
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• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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

• This PR is translated from: Update the introduction of UCASE() and UNHEX() docs-cn#16168
• Other reference link(s): https://docs.pingcap.com/zh/tidb/dev/string-functions

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

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-27 02:10:33.515559944 +0000 UTC m=+927922.263183054: ☑️ agreed by qiancai.

[ti-chi-bot]

@qiancai: Your lgtm message is repeated, so it is ignored.

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[xzhangxian1008]

LGTM

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LGTM

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[lilin90]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #16607.