Clarify documentation on global.json usage

build/Targets/Imports.targets

@@ -476,6 +476,10 @@
       <ShortSdkVersion Condition=" $(dotnetSdkVersion.IndexOf('-')) &lt;= 0 ">$(dotnetSdkVersion)</ShortSdkVersion>
     </PropertyGroup>
 
+    <!-- The $(UsingMicrosoftNETSdk) property is set when we are using a 2.0 SDK or newer (non-legacy). This means
+         we can depend on global.json having enforced the minimum version we require for our build. If it's not
+         set though we need to error out and provide a link to get the correct SDK installed.
+      -->
     <Error 
       Text="The $(ShortSdkVersion) SDK is required to build this repo. It can be install here https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.2.0-preview1-007622/dotnet-sdk-2.2.0-preview1-007622-win-x64.exe"
       Condition="'$(UsingMicrosoftNETSdk)' == '' AND '$(RoslynSdkProject)' == 'true'" />

docs/contributing/Building, Debugging, and Testing on Windows.md

@@ -23,7 +23,7 @@ If a stack trace is displayed on .NET Framework older than 4.7.1 (e.g. by xUnit
 1. [Visual Studio 2017 Update 3](https://www.visualstudio.com/vs/)
     - Ensure C#, VB, MSBuild, .NET Core and Visual Studio Extensibility are included in the selected work loads
     - Ensure Visual Studio is on Version "15.3" or greater
-1. [.NET Core SDK 2.2](https://www.microsoft.com/net/download/core) (if you don't see the 2.1 SDK binaries there yet, the current previews are: [Windows x64 installer](https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.2.0-preview1-007622/dotnet-sdk-2.2.0-preview1-007622-win-x64.exe), [Windows x86 installer](https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.2.0-preview1-007622/dotnet-sdk-2.2.0-preview1-007622-win-x86.exe))
+1. [.NET Core SDK 2.2](https://www.microsoft.com/net/download/core) (if you don't see the 2.2 SDK binaries there yet, the current previews are: [Windows x64 installer](https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.2.0-preview1-007622/dotnet-sdk-2.2.0-preview1-007622-win-x64.exe), [Windows x86 installer](https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.2.0-preview1-007622/dotnet-sdk-2.2.0-preview1-007622-win-x86.exe))
 1. [PowerShell 3.0 or newer](https://docs.microsoft.com/en-us/powershell/scripting/setup/installing-windows-powershell). If you are on Windows 10, you are fine; you'll only need to upgrade if you're on Windows 7. The download link is under the "upgrading existing Windows PowerShell" heading.
 1. Run Restore.cmd
 1. Open Roslyn.sln