docs: add summary lines to 10 type/validity operation docstrings

[TaiSakuma]

Summary

Rolls the pilot from #3946 out to the 10 operations in the Type / validity / equality batch of #3980:

almost_equal, array_equal, is_valid, validity_error, is_categorical, is_tuple, categories, enforce_type, values_astype, strings_astype.

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.almost_equal	Returns True if two arrays are equal within the given tolerances and options.
ak.array_equal	Returns True if two arrays have the same shape and elements.
ak.is_valid	Returns True if the array has no structural errors and False otherwise.
ak.validity_error	Returns an error message if the array has a structural error, or empty if valid.
ak.is_categorical	Returns True if the array is categorical.
ak.is_tuple	Returns True if a record, or the outermost record of an array, is a tuple.
ak.categories	Returns the categories of a categorical array.
ak.enforce_type	Returns an array whose structure is modified to match the given type.
ak.values_astype	Converts all numbers in the array to a new type, leaving the structure untouched.
ak.strings_astype	Converts all strings in the array to a new type, leaving the structure untouched.

Notes for reviewers

• ak.enforce_type's body is one continuous bulleted explanation, so everything sits under a single Returns: header (the ak.concatenate model — no Examples: split that would break the prose). Its doctests are nested inside bullets and were uniformly re-indented by 4 spaces with the bullets; content is byte-identical.
• ak.array_equal delegates to almost_equal with exact settings, so its summary carries no tolerance language, distinguishing the pair.
• Pre-existing issues left verbatim (out of scope here): ak.values_astype has a malformed doctest output in main (datetime64[ms]['), and ak.strings_astype's "See also #ak.numbers_astype" references a non-existent operation.

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 Opus 4.8, 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 Opus 4.8 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.03%. Comparing base (c4aab45) to head (76b7e4c).

Additional details and impacted files

Files with missing lines	Coverage Δ
src/awkward/operations/ak_almost_equal.py	96.36% <ø> (ø)
src/awkward/operations/ak_array_equal.py	100.00% <ø> (ø)
src/awkward/operations/ak_categories.py	100.00% <ø> (ø)
src/awkward/operations/ak_enforce_type.py	83.16% <ø> (ø)
src/awkward/operations/ak_is_categorical.py	100.00% <ø> (ø)
src/awkward/operations/ak_is_tuple.py	100.00% <ø> (ø)
src/awkward/operations/ak_is_valid.py	100.00% <ø> (ø)
src/awkward/operations/ak_strings_astype.py	100.00% <ø> (ø)
src/awkward/operations/ak_validity_error.py	92.85% <ø> (ø)
src/awkward/operations/ak_values_astype.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.almost_equal	True if the two array-like arguments are equal within the given options and tolerances, False otherwise.
ak.array_equal	True if two arrays have the same shape and elements, False otherwise.
ak.is_valid	True if array has no structural errors, False otherwise.
ak.validity_error	An empty string if array is valid, or a string describing the structural error otherwise.
ak.is_categorical	True if array is categorical, False otherwise.
ak.is_tuple	True if array (or its outermost record) is a tuple, False otherwise.
ak.categories	The distinct category values of array (if it is categorical).
ak.enforce_type	An array whose structure is modified to match the given type.
ak.values_astype	An array with every number converted to the given to type, leaving the structure untouched.
ak.strings_astype	An array with every string converted to the given to type, leaving the structure untouched.

[github-actions]

The documentation preview is ready to be viewed at http://preview.awkward-array.org.s3-website.us-east-1.amazonaws.com/PR4135