docs: clarify Weixin media voice behavior

[flute]

Summary

• Clarify that outbound media routing currently sends audio/* as a FILE attachment.
• Clarify that protocol-level VOICE types exist, but the SDK does not currently expose a public sendVoice helper.
• Add a note that sendmessage HTTP success is not sufficient; callers should verify business ret and real client rendering for media.

Why

The protocol type table lists VOICE, but the current outbound send implementation only provides text/image/video/file helpers. Users may assume VOICE is product-ready for native WeChat voice bubbles, while current SDK behavior routes audio MIME files to file attachments.

This documentation change avoids users mistaking "type exists in protocol" for "native voice send is supported".

Related issues:

• VOICE media/item can be accepted by API but does not render as a native WeChat voice bubble #215
• sendmessage ret=-2 during consecutive media sends lacks actionable error details #216

Validation

Docs-only change; no runtime behavior changed.

[NewFuture]

感谢你澄清语音媒体的实际边界。社区版当前面向用户的中英文文档仍未明确说明：audio/* 出站目前会作为文件附件发送，协议中存在 VOICE 类型并不等于已经支持原生微信语音气泡，HTTP 成功也不能替代业务返回值和客户端渲染验证。

如果你愿意，欢迎由你基于社区仓库当前 main 分支提交一个文档 PR：

https://github.com/NewFuture/openclaw-weixin

请适配社区当前的双语文档结构（中文：README.md、docs/guide.md；英文：README_EN.md、docs/guide_EN.md），让中英文表述保持一致，并避免把协议类型描述成已经可用的原生发送能力。本项只需更新与用户行为直接相关的文档，不需要改变运行时代码。

贡献指南：
https://github.com/NewFuture/openclaw-weixin/blob/main/CONTRIBUTING.md

说明：NewFuture/openclaw-weixin 是社区维护发行版，并非腾讯/微信官方版本。社区 PR 会保留你的原作者署名，并链接本 PR 说明来源和适配差异。