PortToTripleSlash might not be working with Microsoft.BCL assemblies

cc @michaelgsharp 

Need to investigate this:
```
PortToTripleSlash  -CsProj C:\repos\runtime\src\libraries\Microsoft.Bcl.Numerics\src\Microsoft.Bcl.Numerics.csproj  -Docs C:\repos\dotnet-api-docs\xml  -IncludedAssemblies "System.Runtime,System.Private.CoreLib,Microsoft.Bcl"  -IncludedNamespaces System
```