docs: Fix default values for unclear types

[thding]

🤔 这个变动的性质是？

• 新特性提交
• 日常 bug 修复
• 站点、文档改进
• 组件样式改进
• TypeScript 定义更新
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他改动（是关于什么的改动？）

🔗 相关 Issue

无

💡 需求背景和解决方案

无

📝 更新日志怎么写？

语言	更新描述
🇺🇸 英文	Modify a document
🇨🇳 中文	修改文档

• 中文（可选）:

☑️ 请求合并前的自查清单

• 文档已补充或无须补充
• 代码演示已提供或无须提供
• TypeScript 定义已补充或无须补充
• Changelog 已提供或无须提供

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View rendered components/form/index.en-US.md
View rendered components/form/index.zh-CN.md

[codecov]

Codecov Report

Merging #19867 into 4.0-prepare will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           4.0-prepare   #19867   +/-   ##
============================================
  Coverage        97.46%   97.46%           
============================================
  Files              288      288           
  Lines             7219     7219           
  Branches          1957     1957           
============================================
  Hits              7036     7036           
  Misses             183      183

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