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

打开 Doc/reference/datamodel.rst 中的 module.file 部分，并根据 issue 的要求检查当前措辞。完成的标准是：文本将该属性描述为可选，指出 str 和通常的绝对路径行为，并说明需要绝对路径的调用方应显式地对其进行规范化。