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

string.Formatter.parse と get_field の上にあるコメントから始め、次に issue にリンクされている public API のレポートと完全なレポートを確認します。報告された各箇所について、private 名が再エクスポートされているかどうかも含め、public function の上にあるコメントに関するルールに照らして確認します。適切な箇所で modern docstring が使用され、意図的に残されたコメントは変更されていない状態を完了とします。リンクされている PR gh-134083、gh-134096、gh-134125 では、関連する作業がすでに対応されています。