gh-152397: Add note to itertools.chain.from_iterable docs re PEP 798

[Twix1288]

This PR adds a short note pointing readers to the inline unpacking-in-comprehension form added by PEP 798, as a more readable alternative for simple flattening on Python 3.15+.

Closes #152397

• Issue: the itertools.chain.from_iterable docs should gain a short note pointing readers PEP 798 alternative #152397

[python-cla-bot]

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± library/itertools.html

[rhettinger]

Hmm, I just added a PR for this 20 minutes before this one was posted: #152454. I will add you as a co-contributor on that PR.

Stylistically, I prefer the other edit as more terse, factual, and letting the reader make their own comparison/contrast without the docs themselves making a value judgment.