fix(docs): Fix merge command docs

[pratyakshsharma]

Description

Fix merge command docs

Motivation and Context

Fix docs

Impact

Test Plan

Contributor checklist

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• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
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Release Notes

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== NO RELEASE NOTE ==

Summary by Sourcery

Documentation:

• Correct the merge command reference documentation to reflect the current behavior and usage.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Updates the MERGE command documentation in presto-docs to correct or clarify the documented syntax/behavior.

File-Level Changes

Change	Details	Files
Corrected and clarified the SQL MERGE command documentation.	Updated the described MERGE syntax and/or examples to match actual engine behavior Fixed inaccurate or misleading descriptions of MERGE semantics and clauses (e.g., WHEN MATCHED/NOT MATCHED behavior, conditions, or target/source references) Adjusted surrounding explanatory text to improve clarity and reduce potential user confusion	presto-docs/src/main/sphinx/sql/merge.rst

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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[steveburnett]

LGTM! (docs)

Pull branch, local doc build, looks good. Thanks!

[steveburnett]

The CI block for docs-only PRs has been fixed in #27285. Could you rebase to the latest master branch? Then we can move forward and merge this PR.

Please add

== NO RELEASE NOTE ==

to the PR description.

[pratyakshsharma]

It is already there @steveburnett . Do you want me to do anything more?

[steveburnett]

It is already there @steveburnett . Do you want me to do anything more?

No, thanks! That looks good.