Add more information about the `runtimes` folder.

In NuGet packages, it is possible to put libraries for different architectures by placing them into a `runtimes` folder.
Documentation about that is here, under the Architecture specific folders section: https://learn.microsoft.com/en-us/nuget/create-packages/supporting-multiple-target-frameworks#architecture-specific-folders

Here is a good example of a package doing so: https://www.nuget.org/packages/Libuv

When publishing an application as multi-platform and framework dependent, and with pinvoke, the pinvoke will find the right lib to use under the right subfolder of `runtimes`.

The documentation does not tell anything about that behavior and the way DllImport seeks for the right architecture under the `runtimes` subfolders.

Can you add that in the documentation?

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#### Document Details

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* ID: 28b4e055-ad4c-cd47-079f-495fdaf0d60c
* Version Independent ID: 5b5aab0f-6879-c6a2-21b5-252819f82272
* Content: [Cross Platform P/Invoke](https://learn.microsoft.com/en-us/dotnet/standard/native-interop/cross-platform)
* Content Source: [docs/standard/native-interop/cross-platform.md](https://github.com/dotnet/docs/blob/main/docs/standard/native-interop/cross-platform.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @saul
* Microsoft Alias: **jekoritz**