Replace `#` comments with modern docstrings when possible

In string.Formatter.{parse,get_field}, we use simple # comments to document methods. While it's picked up by pydoc (through inspect.getcomments()), it's not picked up by IDEs. I suggest we cleanup those occurrences to improve code readability as well. I stumbled upon it by chance so there might be other places that need such cleanup.

[!IMPORTANT]
Only comments that are above a def of a public function should be considered. Comments inside the function body or method body should be considered separately as it could also mean that the public API is incompletely documented on purpose (see https://github.com/python/cpython/issues/134082#issuecomment-2887461014 and following).

• Report including private names: https://gist.github.com/picnixz/d4aa5ff5e82013966dc575f0709c4398#file-full-report-md
• Report for publicly-named API: https://gist.github.com/picnixz/d4aa5ff5e82013966dc575f0709c4398#file-report-md

Both reports parse private files as well as they can be re-exported to a public API. We still need to go through each occurrences to check if they are expected to be exposed or not (and thus the task is NOT an easy task for newcomers).

Linked PRs

• gh-134083
• gh-134096
• gh-134125

先从 string.Formatter.parse 和 get_field 上方的注释开始，然后查看 issue 中链接的 public API 报告和完整报告。根据 public function 上方注释的规则检查每个报告的出现位置，包括 private 名称是否被重新导出。完成的标准是：适当的出现位置使用现代 docstring，同时有意保留的注释保持不变；已链接的 PR gh-134083、gh-134096 和 gh-134125 已经涵盖了相关工作。