Improve reference for fields parameter

[pamelafox]

Description

This PR improves the reference for the VectorizedQuery fields parameter to be clearer, especially about how to specify multiple fields.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes -- Docs only.

[pamelafox]

cc @mattgotteiner

[xiangyan99]

This is auto-generated from https://github.com/Azure/azure-rest-api-specs/blob/main/specification/search/data-plane/Azure.Search/preview/2024-03-01-Preview/searchindex.json#L1866.

Please update the swagger instead. Or the change will be overwritten automatically when we regenerate again.

[pamelafox]

Okay! Azure/azure-rest-api-specs#28483