Document the CopyDebugSymbolFilesFromPackages and CopyDocumentationFilesFromPackages properties

[baronfel]

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In dotnet/sdk#27580, two new properties were added to allow access to a new SDK feature - automatic includes of PDB and XML files for PackageReferences. This is one of our longest-standing community requests.

We should document these new flags and the impact they have in the same location as the other MSBuild project properties: https://docs.microsoft.com/en-us/dotnet/core/project-sdk/msbuild-props