Update Databricks Section of grants.md

[apython1998]

Add bullet point for Databricks specific grants that aren't supported by dbt grants

What are you changing in this pull request and why?

Changing the grants docs for Databricks. Currently rights like READ_METADATA and USAGE are not supported by dbt grant and need to use post-hooks for this

Checklist

• [ ✅ ] Review the Content style guide and About versioning so my content adheres to these guidelines.

[welcome]

Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
We use Markdown and some HTML to write the dbt product documentation. When writing content, you can use our style guide and content types to understand our writing standards and how we organize information in the dbt product docs.
We'll review your contribution and respond as soon as we can. 😄

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[mirnawong1]

thanks for opening this pr @apython1998 , and i appreciate you clarifying this for users! I'll also cc @dataders so he's aware of this change.

will get this merged for you ✨