Document GROUP BY AUTO syntax

[ebyhr]

Description

Follow-up of #18390

Release notes

(x) This is not user-visible or is docs only, and no release notes are required.

[Copilot]

Pull Request Overview

This PR adds documentation for the GROUP BY AUTO syntax to the SQL SELECT documentation. The enhancement documents a new feature that automatically determines grouping keys based on non-aggregated columns in the SELECT clause.

• Adds a new "AUTO" section explaining the GROUP BY AUTO syntax
• Includes a practical example with the shipping table demonstrating automatic grouping
• Shows expected output format for the GROUP BY AUTO query

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[martint]

This needs better phrasing. Maybe something like:

When AUTO is specified, the Trino engine automatically determines the grouping columns instead of requiring them to be listed explicitly. In this mode, any column in the SELECT list that is not part of an aggregate function is implicitly treated as a grouping column.

[martint]

This paragraph describes the query, but does not explain the conclusion that AUTO derives mktsegment as the grouping key since it's not part of any aggregate function. Maybe rephrase to:

This example query calculates the total account balance per market segment. The AUTO clause derives mktsegment as the grouping key, since it is not used in any aggregate function (i.e., sum).