Clarify mongodb services to avoid customer confusion.

[gahl-levy]

PR Summary

This change adds a single link to Azure DocumentDB (with MongoDB compatibility) in the az mongo-db atlas Azure CLI reference note, helping readers distinguish Microsoft-managed MongoDB-compatible services from vendor-managed MongoDB Atlas since the current name "mongodb" leads to ambiguity and customer confusion.

PR Checklist

Descriptive Title: This PR's title is a synopsis of the changes it proposes.

Summary: This PR's summary describes the scope and intent of the change.

[learn-build-service-prod]

PoliCheck Scan Report

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[learn-build-service-prod]

Learn Build status updates of commit 7ec22ca:

✅ Validation status: passed

File	Status	Preview URL	Details
docs-ref-autogen/Latest-version/latest/mongo-db/atlas.yml	✅Succeeded	View (azure-cli-latest)

For more details, please refer to the build report.

[mikefrobbins]

@gahl-levy, This PR targets autogenerated reference content and the changes will be overwritten. The source for this content is located in the Azure CLI source repo.