[drivers][adc] Fix misleading doxygen for rt_adc_voltage

[mnikapl954]

拉取/合并请求描述：(PR description)

为什么提交这份PR (why to submit this PR)

rt_adc_voltage() is documented as "get the adc resolution" and its @return says "adc resolution". That description belongs to resolution APIs, not voltage conversion, and confuses readers of the driver API docs (#9424 related doxygen quality).

你的解决方案是什么 (what is your solution)

Correct the @brief / @return text for rt_adc_voltage so it describes returning a converted voltage value.

Change is limited to doxygen comments in components/drivers/include/drivers/adc.h.

请提供验证的bsp和config (provide the config and bsp)

• BSP: N/A (doxygen comment only)
• Validation: checked function name and nearby ADC APIs in the same header

当前拉取/合并请求的状态 Intent for your PR

• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确 Style guide is adhered to
• 没有垃圾代码 All redundant code is removed
• 所有变更均有原因及合理的 All modifications are justified
• 代码是高质量的 Code in this PR is of high quality

[github-actions]

📌 Code Review Assignment

🏷️ Tag: components

Reviewers: @Maihuanyi

Changed Files (Click to expand)

• components/drivers/include/drivers/adc.h

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📊 Current Review Status (Last Updated: 2026-07-22 16:39 CST)

• ⌛ @Maihuanyi Pending Review

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[github-actions]

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为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
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