docs(cosmosdb-preview): use Azure DocumentDB in copy help

[Patty (patty-chow)]

Summary

Use the current Azure DocumentDB product name in the cosmosdb-preview extension's copy-destination help. This is the upstream source of the legacy product name in generated Azure CLI reference pages.

Changes are limited to four user-facing text substitutions across _params.py and _help.py: argument-group name, destination description, destination short summary, and example title. Existing --dest-mongo-vcore, dest_mongo_vcore, AddMongoVCoreCollectionAction, command examples, RU-product references, behavior, and historical release notes are preserved.

Product naming reference: https://learn.microsoft.com/azure/documentdb/faq

Related command

az cosmosdb copy create --dest-mongo-vcore

Validation

• Python compilation of both edited files passes (syntax validation; no Azure operations executed).
• Exact reverse-substitution check proves no other bytes changed.
• All CLI flag spellings are unchanged.
• No generated documentation files or vendored SDKs edited.
• azdev is not installed locally; style/index checks are not claimed.

General Guidelines

• Run azdev style cosmosdb-preview (pending repository validation).
• Run python scripts/ci/test_index.py -q (not run locally).
• Extension release/version decision pending maintainer review.

Extension publication

Draft: this user-requested documentation correction does not intentionally publish a new extension release. setup.py, HISTORY.rst, and src/index.json are unchanged. Please determine the appropriate version/changelog treatment before making this PR ready for merge. No merge or automatic publication requested.

AI assistance was used to prepare and validate these narrowly scoped help-text changes.

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[Aditya Pujara (a0x1ab)]

/azp run

[azure-pipelines]

Azure Pipelines:
Successfully started running 2 pipeline(s).