docs: 改进 skeleton 首次加载数据使用场景描述

[hi-rustin]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

💡 Background and solution

我觉得原来的描述很容易让人误解，这样修改可能会好点。

📝 Changelog

Language	Changelog
🇺🇸 English	Improved skeleton first load data using scene description
🇨🇳 Chinese	改进 skeleton 首次加载数据使用场景描述

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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View rendered components/skeleton/index.zh-CN.md

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