Itertool docs: Minor clarifications, wording tweaks, spacing, and active voice.

[rhettinger]

• Improve groupby example to avoid redundant input data which distracts from the grouping by length.
• Eliminate confusing modifier: module functions -> functions
• Active voice for the accumulate examples.
• Higher level use case driven description of chain.
• Add line spacing to the cycle equivalent code to highlight the two phases.
• Add line spacing to the filterfalse equivalent code to separate the primary logic from the predicate setup.
• In the chain description, we had issues with the word iterable which matches the parameter name versus iterator which is the only case where it makes sense to talk about running to exhaustion. Instead, use the high-level word input which is clearer.
• Add line spacing to the pairwise equivalent code to visually separate the setup code from the main loop logic.

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📚 Documentation preview 📚: https://cpython-previews--124690.org.readthedocs.build/

[DinoV]

LGTM!

[miss-islington-app]

Thanks @rhettinger for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @rhettinger, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 2e155536caf8a090c06d62dd92647abc62362463 3.13

[bedevere-app]

GH-125148 is a backport of this pull request to the 3.13 branch.