Document that PPL fetch_size limits aggregation buckets

[ahkcs]

Description

fetch_size is applied to the final rows of a PPL query, so on an aggregating query it limits the number of aggregation buckets returned rather than sampling documents. The existing docs described it only as "limits the number of rows returned," which reads like DSL size — where aggregation results come back in full.

This adds a short section covering:

• fetch_size is equivalent to appending | head <fetch_size>, so for stats/timechart/top/rare/patterns ... mode=aggregation it truncates buckets.
• Aggregate values stay correct — unlike DSL terminate_after, which undercounts because documents are never collected. But unlike DSL size, aggregation results are truncated. It matches neither analogue exactly, which is what made the behavior easy to misread.
• Bucket counts grow with the queried time range rather than data volume, so a fixed limit truncates unexpectedly on wide ranges.

Raised in review of opensearch-project/OpenSearch-Dashboards#12511, where this behavior caused Discover to silently drop buckets from PPL visualizations.

Issues Resolved

None; documentation gap found during review of OpenSearch-Dashboards#12511.

Check List

• New functionality has been documented.
• Commits are signed per the DCO using --signoff

Docs-only change. docs/user/interfaces/ is not covered by the doctest suite, so no queries here are executed as assertions; the example values were confirmed by hand against a local 3.8.0-SNAPSHOT cluster.

[github-actions]

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[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Standardize code example formatting The code examples have inconsistent spacing around the query and comment separator. The first line uses multiple spaces before ->, while the second uses spaces around (fetch_size=3) and before ->. Standardize the formatting for better readability. docs/user/interfaces/endpoint.rst [338-339] -source=logs | stats count() by extension -> 6 buckets -source=logs | stats count() by extension (fetch_size=3) -> first 3 buckets only +source=logs | stats count() by extension -> 6 buckets +source=logs | stats count() by extension (fetch_size=3) -> first 3 buckets only Suggestion importance[1-10]: 3 __ Why: The suggestion correctly identifies minor spacing inconsistencies in the code examples. However, the impact is minimal as both formats are readable, and the improved_code still shows inconsistent spacing (extra spaces after extension vs. after (fetch_size=3)). This is a minor stylistic improvement.	Low

[penghuo]

As this is PPL limitations doc, Remove following sentence

SQL queries with fetch_size parameter (cursor-based pagination). Note: