gh-148552: Document the loop argument in ensure_future() method

[manoj-km24]

Document loop argument in Doc/library/asyncio-future.rst under the ensure_future() method.

• Issue: Document loop argument in ensure_future method #148552

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📚 Documentation preview 📚: https://cpython-previews--148553.org.readthedocs.build/

[picnixz]

I do not think it is necessary to document it only in one function. Just document it at the toplevel of the module somehow brcause it is not the only function taking n optionsal loop.

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