Update outdated overflow/size guidance in libcudf developer guide

[PointKernel]

Description

This PR fixes outdated content in the "libcudf does not introspect data" section of the C++ developer guide.

Two changes:

• The "Things that libcudf should not validate" list cited a "2GB size limit" example that dates back to the original policy draft (Initial draft of policies and guidelines for libcudf usage. #11853), when a strings column's character buffer was capped at `size_type` bytes. Large strings support (64-bit offsets) removed that limit, and the linked `#cudfsize_type` anchor actually documents the row/element count limit, not a byte size. Reworded to the still-valid `size_type` row count limit.
• Added a short paragraph clarifying the policy describes libcudf's default behavior, and pointing to the opt-in overflow-aware variants that now exist: the `SUM_OVERFLOW` aggregation and the overflow-checking AST arithmetic operators (`ADD_OVERFLOW`, `SUB_OVERFLOW`, and similar). Without this, the section reads as if libcudf has no overflow story at all, which is no longer true.

Docs only; no code or behavior changes.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 23ef5e5 and 8c1ad71.

📒 Files selected for processing (1)

• cpp/doxygen/developer_guide/DEVELOPER_GUIDE.md

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📝 Walkthrough

Summary by CodeRabbit

• Documentation
  • Clarified guidance on output-size limits, replacing the older “2GB size” wording with a more precise explanation of row-count overflow behavior.
  • Added notes that overflow-aware behavior is only available through opt-in API variants and selected arithmetic operations.

Walkthrough

This PR updates the developer guide documentation to clarify that libcudf does not validate output-size overflow against the size_type row-count limit, and adds documentation about opt-in overflow-aware APIs such as SUM_OVERFLOW and overflow-checking AST operators.

Changes

Developer guide documentation update

Layer / File(s)	Summary
Overflow validation wording update cpp/doxygen/developer_guide/DEVELOPER_GUIDE.md	Replaces the "2GB size" phrasing with size_type row-count limit language and documents opt-in overflow-aware variants (SUM_OVERFLOW, overflow-checking AST arithmetic operators).

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested labels: libcudf, improvement, non-breaking

Suggested reviewers: vyasr, mhaseeb123, davidwendt

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the documentation update to overflow/size guidance in the libcudf developer guide.
Description check	✅ Passed	The description matches the documented changes and clearly explains the outdated size guidance and overflow-aware API updates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[PointKernel]

/merge