`Protocol` should be documented as using `ABCMeta` as metaclass

Documentation

Came up in #127648.

Currently the documentation for typing.Protocol doesn't mention that Protocol has ABCMeta in its metaclasses (indirectly via _ProtocolMeta) and that its methods can (should?) be marked with @abstractmethod. In fact, neither the string "ABCMeta" nor "abstractmethod" appear in the documentation for the typing module.

I also suggest to add @abstractmethod to the examples using a Protocol as that is probably a best practice.

Finally, the documentation for Protocol suggests to see PEP 544 for more details. I don't think this is particularly helpful. All information relevant for users should be in the main documentation. PEP 544 should only be suggested as a historical document. (And I don't suggest to directly link to the typing spec, either, although a general link at the top of the page would be useful.)

Issue にリンクされている typing.Protocol のドキュメントから始め、その指針を PEP 544 と比較します。Protocol の例と説明を確認し、要求されている ABCMeta、abstractmethod、および参照の網羅性を確認します。関連するユーザー向けの詳細が文書化され、PEP の参照が歴史的な文脈として位置付けられていれば完了です。