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

先從 string.Formatter.parse 和 get_field 上方的註解開始，然後檢視 issue 中連結的 public API 報告和完整報告。根據 public function 上方註解的規則檢查每個回報的出現位置，包括 private 名稱是否被重新匯出。完成的標準是：適當的出現位置使用現代 docstring，同時有意保留的註解維持不變；已連結的 PR gh-134083、gh-134096 和 gh-134125 已涵蓋相關工作。