gh-148091: asyncio : clarify Future.cancel(msg) behaviour

[manoj-k-m]

The msg parameter of Future.cancel() is currently undocumented.

This change clarifies that the optional msg argument is passed as the argument to asyncio.CancelledError raised when
a cancelled Future is awaited, and that it can be of any object.

Fixes #148091

• Issue: Document how the msg parameter for asyncio.futures.Future.cancel is used #148091

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[manoj-k-m]

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[ZeroIntensity]

Please update the asyncio documentation, not the docstring. We don't generate the documentation from docstrings.

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