{Azure Bastion} documentation fix and update preview flag

[aavalang]

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

Related command

General Guidelines

• [x] Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• [x] Have you run python scripts/ci/test_index.py -q locally? (pip install wheel==0.30.0 required)
• [ ] My extension version conforms to the Extension version schema

For new extensions:

• [x] 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.