Add comments explaining .NET Standard facade injection

src/Tasks/Microsoft.NET.Build.Extensions.Tasks/msbuildExtensions/Microsoft/Microsoft.NET.Build.Extensions/Microsoft.NET.Build.Extensions.NETFramework.targets

@@ -76,11 +76,25 @@ Copyright (c) .NET Foundation. All rights reserved.
            targets. -->
       <Reference Remove="%(_NETStandardLibraryNETFrameworkLib.FileName)" />
 
+      <!-- Put each facade assembly in two separate items: Reference with Private set to false, which means it won't be
+           copied locally, and ReferenceCopyLocalPaths, which will be copied locally.  The reason for this split is to
+           workaround https://github.com/dotnet/core-setup/issues/2981 by ensuring that the facades are written
+           to the deps.json with a type of "referenceassembly" instead of "reference".
+           
+           The exception is netfx.force.conflicts.dll, which shouldn't be copied local.  So it isn't included in
+           ReferenceCopyLocalPaths.
+           
+           When we add the Reference items, we use the simple name as the ItemSpec, and refer to the full path
+           to the DLL via the HintPath metadata.  This is so that if we're replacing a simple Reference,
+           the OriginalItemSpec of the resolved reference will match to the Reference that was replaced,
+           and VS won't show a warning icon on the reference.  See https://github.com/dotnet/sdk/issues/1499
+           -->
+
       <Reference Include="@(_NETStandardLibraryNETFrameworkLib.FileName)">
         <HintPath>%(_NETStandardLibraryNETFrameworkLib.Identity)</HintPath>
         <Private>false</Private>
       </Reference>
-      <!-- netfx.force.conflicts is only needed at compile time. -->
+
       <ReferenceCopyLocalPaths Include="@(_NETStandardLibraryNETFrameworkLib)" Condition="'%(FileName)' != 'netfx.force.conflicts'">
         <Private>false</Private>
       </ReferenceCopyLocalPaths>