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

Comienza con los comentarios situados encima de string.Formatter.parse y get_field, y después revisa los informes de la API pública y los informes completos enlazados en el issue. Comprueba cada aparición indicada según la regla para los comentarios situados encima de las funciones públicas, incluido si los nombres privados se vuelven a exportar. Se considera terminado cuando las apariciones correspondientes utilizan docstrings modernos, mientras que los comentarios conservados intencionadamente permanecen sin cambios; los PR enlazados gh-134083, gh-134096 y gh-134125 ya cubren trabajo relacionado.