Add migration note for Swashbuckle CLI to built-in OpenAPI support (#36512)

* Initial plan

* Add migration note for Swashbuckle CLI users to built-in OpenAPI support

Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>

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Added link for package

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Clarified Interactive UI.

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Add NuGet link for package

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>
Co-authored-by: Wade Pickett <wpickett@microsoft.com>

Author: Copilot

aspnetcore/fundamentals/openapi/aspnetcore-openapi.md

@@ -1,11 +1,12 @@
 ---
 title: Generate OpenAPI documents
+ai-usage: ai-assisted
 author: captainsafia
 description: Learn how to generate and customize OpenAPI documents in an ASP.NET Core app.
-ms.author: safia
 monikerRange: '>= aspnetcore-6.0'
+ms.author: safia
 ms.custom: mvc
-ms.date: 12/10/2025
+ms.date: 12/17/2025
 uid: fundamentals/openapi/aspnetcore-openapi
 ---
 # Generate OpenAPI documents
@@ -183,6 +184,11 @@ You can customize the <xref:Microsoft.AspNetCore.OpenApi.OpenApiOptions.ShouldIn
 
 ## Generate OpenAPI documents at build time
 
+> [!NOTE]
+> Starting with .NET 9, ASP.NET Core includes built-in OpenAPI support. The [`Microsoft.AspNetCore.OpenApi`](https://www.nuget.org/packages/Microsoft.AspNetCore.OpenApi) package provides OpenAPI document generation at runtime, and adding the [`Microsoft.Extensions.ApiDescription.Server` ](https://www.nuget.org/packages/Microsoft.Extensions.ApiDescription.Server) package enables build-time document generation. 
+>
+>ASP.NET Core generates OpenAPI documents only. Interactive UIs such as **Swagger UI** or **Scalar** are not included by default and must be added separately. For guidance on using these UI options, see <xref:fundamentals/openapi/using-openapi-documents>.
+
 In typical web apps, OpenAPI documents are generated at runtime and served via an HTTP request to the app server.
 
 In some scenarios, it's helpful to generate the OpenAPI document during the app's build step. These scenarios include generating OpenAPI documentation that is: