Improve Microsoft.Net.Sdk.Compilers.Toolset package description (dotnet#42212)

Author: marcpopMSFT

src/Microsoft.Net.Sdk.Compilers.Toolset/Microsoft.Net.Sdk.Compilers.Toolset.csproj

@@ -2,7 +2,25 @@
 
   <PropertyGroup>
     <TargetFramework>$(SdkTargetFramework)</TargetFramework>
-    <Description>Transport package for Microsoft.Net.Compilers.Toolset.Framework assemblies. For internal use only.</Description>
+    <Description><![CDATA[
+      Transport package for Microsoft.Net.Compilers.Toolset.Framework assemblies. For internal use only:
+      do not reference this package directly as it is explicitly not supported in that fashion.
+
+      This package is automatically downloaded when your MSBuild version does not match your SDK version.
+      Then the package is used to build your project with the compiler version matching your SDK version
+      instead of the one bundled with MSBuild.
+
+      If you want to download this package using the dotnet CLI, you can use the following command:
+
+      ```
+      dotnet restore /p:BuildWithNetFrameworkHostedCompiler=true
+      ```
+
+      If you want to disable automatic download and use of this package, set property `BuildWithNetFrameworkHostedCompiler` to `false`
+      (for example, in your `.csproj` or `Directory.Build.props` file).
+      However, be aware that you will be building with mismatched compiler and SDK versions.
+      That is explicitly not supported and can lead to errors (especially with analyzers and source generators).
+    ]]></Description>
     <IsPackable>true</IsPackable>
     <IsShippingPackage>true</IsShippingPackage>
     <IncludeBuildOutput>false</IncludeBuildOutput>