Microsoft.Extensions.DependencyModel should be documented

There's a NuGet package [Microsoft.Extensions.DependencyModel](https://www.nuget.org/packages/Microsoft.Extensions.DependencyModel/) for reading *.deps.json* files at runtime. It is built from sources in core-setup now [runtime](https://github.com/dotnet/runtime/tree/main/src/libraries/Microsoft.Extensions.DependencyModel) repository. Currently there's no documentation for it. This package can be useful for developers who use Roslyn API to compile code targeting .NET Core (example: https://github.com/dotnet/core/issues/2082#issuecomment-442713181).

If this is supported library, i think it should be documented in .NET API Docs

**Update:** Now this library is shown in , but all descriptions are empty. Leaving this issue open to track filling API doc desriptions.