Clarify module.__file__ documentation in the data model reference

Documentation

The module.__file__ section in Doc/reference/datamodel.rst has two problems:

• it says "Both attributes should be a str" even though the section is describing a single attribute
• it does not clearly explain the path form readers should expect from __file__

The docs should say that __file__ is optional, should be a str when set, and is usually an absolute path for modules loaded from a file by the import system, while callers that require an absolute path should normalize it explicitly.

Linked PRs

• gh-150273

Doc/reference/datamodel.rst の module.file セクションを開き、issue の要件に照らして現在の文言を確認してください。テキストがこの属性をオプションとして説明し、str と通常の絶対パスの動作を明記し、絶対パスが必要な呼び出し元は明示的に正規化すべきだと述べていれば完了です。