[doc] State clearly that open() 'file' param is "name" attr of the result

[GrigoriiKashchenko]

Summary

Update documentation to clearly state that the file argument passed to open() is stored as the name attribute of the returned file object.

Changes Made

1. Doc/library/functions.rst: Added explicit documentation for the open() function
2. Doc/library/io.rst: Updated documentation for FileIO.name and added missing IOBase.name attribute

Key Clarifications

• The name attribute stores exactly what was passed as the file argument
• It can be string, bytes, integer file descriptor, or other representation
• This behavior is preserved for backward compatibility
• Added .. versionchanged:: 3.13 directive to document the clarification

Testing

• Verified documentation builds correctly
• Behavior matches existing Python implementation
• Cross-references between functions.rst and io.rst are consistent

Closes gh-62734

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[python-cla-bot]

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• GrigoriiKash96@yandex.ru