update description of datamigration sql-db create

[yadongyaly]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

Related command

datamigration sql-db create

Add description in "help".
If the customer does not have table in the target database, suggest them to use "az datamigration sql-server-schema" command to do schema migration firstly.

Checklist

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your pull request is merged into main branch, a new pull request will be created to update src/index.json automatically.
You only need to update the version information in file setup.py and historical information in file HISTORY.rst in your PR but do not modify src/index.json.

[azure-client-tools-bot-prd]

️✔️Azure CLI Extensions Breaking Change Test

️✔️Non Breaking Changes

[azure-client-tools-bot-prd]

Hi @yadongyaly,
Please write the description of changes which can be perceived by customers into HISTORY.rst.
If you want to release a new extension version, please update the version in setup.py as well.

[yonzhan]

Thank you for your contribution! We will review the pull request and get back to you soon.

[yadongyaly]

Hi @yonzhan @evelyn-ys @calvinhzy may I get a review on this tiny PR? The change is only adding more descriptions.