docs: add summary lines to 5 math/numeric operation docstrings

[TaiSakuma]

Summary

Rolls the pilot from #3946 out to the 5 operations in the Math / numeric batch of #3980:

round, real, imag, angle, isclose.

Each docstring now has:

• A one-line summary on the opening """.
• Returns: and Examples: section headers (the latter only where examples exist).

Original body text is preserved; only structural edits are applied. The summary lines were reviewed against the checklist posted in #3980.

Summary lines

Operation	Summary
ak.round	Rounds each array element to the given number of decimals.
ak.real	Returns the real components of the given array elements.
ak.imag	Returns the imaginary components of the given array elements.
ak.angle	Returns the counterclockwise angle of each complex element in radians or degrees.
ak.isclose	Returns a boolean array of element-wise approximate-equality between two arrays.

Notes for reviewers

• Pre-existing bug surfaced (left verbatim in the body): ak.round's narrative reads "Returns the real components of the given array elements." — a copy-paste from ak_real.py. The implementation rounds (nplike.round(data, decimals)), and the new summary describes that correctly. The wrong body sentence is preserved per the no-rewrite rule; happy to fix it here or in a follow-up if you prefer.
• ak.isclose and the NumPy-link sentences stay in the bodies (URLs are not allowed in summaries); the summaries describe the effect instead.
• ak.angle's "in radians or degrees" covers the deg argument.

Test plan

• pre-commit passes locally on the modified files (ruff check, ruff format, codespell, mypy).
• Mechanical validation: code outside docstrings byte-identical; Args: blocks and doctests byte-identical; no original narrative text lost.
• Sphinx docs build cleanly on CI.
• Rendered docstrings spot-checked in the docs preview.

Part of #3980. Draft for initial review; will mark ready once CI passes and the preview is checked.

AI assistance disclosure

Drafted with Claude Code: summaries drafted by Claude Sonnet 4.6, adversarially verified against the implementations by independent Claude Opus 4.8 agents, mechanically validated (docstring-only changes, original text preserved), and reviewed by a human before submission.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.74%. Comparing base (722a1b9) to head (9268dd2).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/awkward/operations/ak_angle.py	100.00% <ø> (ø)
src/awkward/operations/ak_imag.py	100.00% <ø> (ø)
src/awkward/operations/ak_isclose.py	96.42% <ø> (ø)
src/awkward/operations/ak_real.py	100.00% <ø> (ø)
src/awkward/operations/ak_round.py	100.00% <ø> (ø)

[TaiSakuma]

Follow-up per @ikrommyd's review on #4130: the Returns: sections now describe the returned object (a noun phrase) rather than restating the operation. Final Returns: lines for this batch:

Returns

Operation	Returns
ak.round	An array with each element rounded to the given number of decimals.
ak.real	The real components of the given array elements.
ak.imag	The imaginary components of the given array elements.
ak.angle	The counterclockwise angle from the positive real axis on the complex plane in the range (-pi, pi], with dtype as a float.
ak.isclose	An array of booleans, True where a and b are approximately equal within the given tolerances.

[github-actions]

The documentation preview is ready to be viewed at http://preview.awkward-array.org.s3-website.us-east-1.amazonaws.com/PR4133