Document XML doc comment support in OpenAPI

[captainsafia]

Starting in .NET 10 Preview 2, the Microsoft.AspNetCore.OpenApi package supports XML document generation by default in its implementation. We should document this behavior.

I've created https://github.com/captainsafia/aspnet-openapi-xml with a sample app and some content that can be the source for documentation on this front.

We don't need to include the implementation details in the doc but the FAQ from the README would be helpful to include.

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Associated WorkItem - 422235

[Rick-Anderson]

@captainsafia remind me of the NuGet source to use for packages not yet pushed to https://api.nuget.org/v3/index.json

Unable to find package Microsoft.AspNetCore.OpenApi with version (>= 10.0.0-preview.2.25161.17)

[captainsafia]

@captainsafia remind me of the NuGet source to use for packages not yet pushed to https://api.nuget.org/v3/index.json

Unable to find package Microsoft.AspNetCore.OpenApi with version (>= 10.0.0-preview.2.25161.17)

You can use the dotnet10 nuget feeds referenced in this README.