add detail desc for String Function SUBSTRING_INDEX () and TO_BASE64

[qiancai]

Fix pingcap/docs-cn#15950

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

• This PR is translated from: add detail desc for String Function SUBSTRING_INDEX () and TO_BASE64 docs-cn#16112, fix desc for String Function SUBSTRING_INDEX () and TO_BASE64 docs-cn#16514
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Do your changes match any of the following descriptions?

• Delete files
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Oreoxmt]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-19 07:42:32.276458894 +0000 UTC m=+256641.024082006: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

[ti-chi-bot]

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