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

開啟 Doc/reference/datamodel.rst 中的 module.file 區段，並根據 issue 的要求檢視目前的措辭。完成的標準是：文字將該屬性描述為可選，指出 str 和通常的絕對路徑行為，並說明需要絕對路徑的呼叫端應明確地將其正規化。