docs: add summary lines to 3 *_like operation docstrings

[TaiSakuma]

Summary

Rolls the pilot from #3946 out to the 3 operations in the Likes batch of #3980:

full_like, ones_like, zeros_like.

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.full_like	Returns an array with the same structure as the input, filled with a given value.
ak.ones_like	Returns an array with the same structure as the input, filled with ones.
ak.zeros_like	Returns an array with the same structure as the input, filled with zeros.

Notes for reviewers

• All three originals began "This is the equivalent of NumPy's np.X_like…", which cannot serve as a verb-first summary; the NumPy-equivalence sentence is preserved verbatim in each body.
• The three summaries form a tight family differing only in the fill description; full_like's source line sits exactly at the 88-char budget.

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 (1a35244).

Additional details and impacted files

Files with missing lines	Coverage Δ
src/awkward/operations/ak_full_like.py	98.36% <ø> (ø)
src/awkward/operations/ak_ones_like.py	93.75% <ø> (ø)
src/awkward/operations/ak_zeros_like.py	94.73% <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.full_like	An array with the same structure as array, with every value replaced by fill_value.
ak.ones_like	An array with the same structure as array, with every value replaced by one.
ak.zeros_like	An array with the same structure as array, with every value replaced by zero.

[github-actions]

The documentation preview is ready to be viewed at http://preview.awkward-array.org.s3-website.us-east-1.amazonaws.com/PR4131