docs: 完善 Legend itemFormatter 文档

[huaxiabuluo]

Checklist

• npm test passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Description of change

[gemini-code-assist]

Summary of Changes

Hello @huaxiabuluo, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

本次拉取请求主要聚焦于完善 F2 图表库中 Legend（图例）组件的文档。通过对 itemFormatter 和 valuePrefix 属性的说明进行补充和澄清，旨在帮助开发者更清晰地理解图例项的格式化逻辑，特别是 name 和 value 的组合方式，从而提高文档的准确性和易用性。

Highlights

• 更新 Legend itemFormatter 文档: 明确了 itemFormatter 的返回值将附加在自动显示的 name 之后，以避免混淆。
• 完善 valuePrefix 属性说明: 为 valuePrefix 属性添加了默认值 ': '，并详细说明了最终的渲染格式为 ${name}: ${value}。
• 新增重要提示: 在示例代码下方添加了一个显眼的提示，强调 itemFormatter 只需返回值部分，因为 name 会自动显示。

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[gemini-code-assist]

Code Review

本次 PR 旨在完善 itemFormatter 的文档，所做的更改方向是正确的。但我发现几处描述可以更精确，以避免用户对 itemFormatter、valuePrefix 和 name 之间如何交互产生困惑。我提供了一些建议来使文档更加清晰明确。

[github-actions]

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
f2/dist/index.js	=	1,706.38 kB	1,706.38 kB	=	323.03 kB	323.03 kB
f2/dist/index.min.js	=	532.93 kB	532.93 kB	=	160.21 kB	160.21 kB

Significant size changes

Includes any change greater than 0.2%:
(No significant changes)

Generated by 🚫 dangerJS against 8f5381c