Clarify changelog guidance to prevent incorrect release notes

[an-cor]

Description

This PR updates the changelog contribution guidance in CONTRIBUTING.md to reduce the chance of stale or placeholder changelog entries producing incorrect release notes.

Issue #2970 describes a release-process problem where release notes can become incorrect when CHANGELOG.md exists but is not kept up to date. The previous guidance instructed contributors to add changelog entries with dummy pull request information and update them later. This PR removes that wording and instead asks contributors to reference the real pull request and verify the entry before merge.

Related Issue

Addresses part of #2970.

What Changed

• Added a note that release notes are generated from the changelog, so stale or inaccurate entries can result in incorrect release notes.
• Removed guidance that encouraged dummy pull request information.
• Clarified that contributors should verify the changelog entry references the correct PR and accurately describes the change before merge.

Testing

This is a documentation-only change.

Validation performed:

git diff --check HEAD~1 HEAD

No YAML validation was needed because this PR only changes CONTRIBUTING.md and does not modify any workflow files.

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[github-actions]

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