DOC: clarify list-like conversion in DataFrame broadcasting behavior

[LarytheLord]

Summary

• clarify that list-like RHS inputs in DataFrame binary ops are converted to Series
• document that this conversion aligns by column position
• add a short example where list elements are ndarray objects, showing why object-valued cells can appear

Issue

Closes #18857

Notes

This is a docs-only clarification in user_guide/basics.rst under the existing "Matching / broadcasting behavior" section.

[LarytheLord]

Follow-up pushed in 24f257b699 to fix docs CI: I replaced the array-of-arrays RHS example (which raises on current pandas) with a scalar list example that still demonstrates positional list-like alignment to columns.

[LarytheLord]

CI note: the latest failing Doc Build and Upload run (22445955642) is failing during website context generation due an external GitHub API timeout (requests.exceptions.ConnectTimeout to api.github.com/users/TomAugspurger), not from a docs syntax/content error in this PR.

I can push/rebase if needed, but from the traceback this looks like transient network flakiness in the website build step.

[LarytheLord]

CI note for Doc Build and Upload: the failing run is timing out on outbound GitHub API calls while building the website context (ConnectTimeoutError to api.github.com), which looks like transient network infra rather than this docs change. Happy to rebase/re-run if maintainers want a fresh run.

[LarytheLord]

CI note: the failing Doc Build and Upload job appears unrelated to this PR content.

The failure occurs while building the website context and is a network timeout to GitHub API:

• requests.exceptions.ConnectTimeout
• api.github.com/users/TomAugspurger
• run: https://github.com/pandas-dev/pandas/actions/runs/22445955642/job/65000331526

I don't have permissions to rerun workflows in this repo. If a maintainer can rerun that failed job, this should confirm whether it was transient infra/network.

[LarytheLord]

Rebased this branch onto current main and force-pushed (d9c14b2) to retrigger a fresh doc build run after the previous GitHub API timeout. No behavioral/content change beyond the existing docs fix.