[comment][libc] add some comments for functions in posix_signal

[ddAkira]

拉取/合并请求描述：(PR description)

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为什么提交这份PR (why to submit this PR)

#7426

There are missing comments for functions in posix_signal.c.

你的解决方案是什么 (what is your solution)

I add the comments based on posix standard and the implement of the functions.

在什么测试环境下测试通过 (what is the test environment)

No need.

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 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：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 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，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用formatting等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification

[CLAassistant]

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[mysterywolf]

请按照上面的评论签署CLA

[mysterywolf]

格式有点问题 请格式化代码：https://github.com/mysterywolf/formatting