Document compiler package usage (#39272)

Cleaning up the documentation on our compiler toolset packages to make
the supported use cases explicitly clear. Previously this information
was only avaliable in scattered issues throughout the repsitory
([example](https://github.com/dotnet/roslyn/issues/38312)).

The package description is likely the best place for this information.

src/NuGet/Microsoft.NETCore.Compilers/Microsoft.NETCore.Compilers.Package.csproj

@@ -8,6 +8,7 @@
     <IncludeBuildOutput>false</IncludeBuildOutput>
     <DevelopmentDependency>true</DevelopmentDependency>
     <PackageDescription>
+      Note: This package is deprecated. Please use Microsoft.Net.Compilers.Toolset instead
       CoreCLR-compatible versions of the C# and VB compilers for use in MSBuild.
     </PackageDescription>
     <TargetsForTfmSpecificContentInPackage>$(TargetsForTfmSpecificContentInPackage);_GetFilesToPackage</TargetsForTfmSpecificContentInPackage>

src/NuGet/Microsoft.Net.Compilers.Toolset/Microsoft.Net.Compilers.Toolset.Package.csproj

@@ -9,9 +9,14 @@
     <DevelopmentDependency>true</DevelopmentDependency>
     <PackageDescription>
       .NET Compilers Toolset Package.
-      Referencing this package will cause the project to be built using the specific version of the C# and Visual Basic compilers contained in the package, as opposed to any system installed version.
+      Referencing this package will cause the project to be built using the C# and Visual Basic compilers contained in the package, as opposed to the version installed with MSBuild.
+
+      This package is primarily intended as a method for rapidly shipping hotfixes to customers. Using it as a long term solution for providing newer compilers on older MSBuild installations is explicitly not supported. That can and will break on a regular basis.
+
+      The supported mechanism for providing new compilers in a build enviroment is updating to the newer .NET SDK or Visual Studio Build Tools SKU.
+
+      This package requires either MSBuild 16.3 and .NET Desktop 4.7.2+ or .NET Core 2.1+
 
-      This package requires MSBuild 15.0 and either .NET Destkop 4.7.2 or .NET Core 2.1
       $(RoslynPackageDescriptionDetails)
     </PackageDescription>
     <TargetsForTfmSpecificContentInPackage>$(TargetsForTfmSpecificContentInPackage);_GetFilesToPackage</TargetsForTfmSpecificContentInPackage>

src/NuGet/Microsoft.Net.Compilers/Microsoft.Net.Compilers.Package.csproj

@@ -8,10 +8,13 @@
     <IncludeBuildOutput>false</IncludeBuildOutput>
     <DevelopmentDependency>true</DevelopmentDependency>
     <PackageDescription>
+      Note: This package is deprecated. Please use Microsoft.Net.Compilers.Toolset instead
+
       .NET Compilers package.
-      Referencing this package will cause the project to be built using the specific version of the C# and Visual Basic compilers contained in the package, as opposed to any system installed version.
+      Referencing this package will cause the project to be built using the C# and Visual Basic compilers contained in the package, as opposed to the version installed with MSBuild.
+
+      The tools in this package require .NET Framework 4.7.2+ 
 
-      This package can be used to compile code targeting any platform, but can only be run using the desktop .NET 4.7.2+ Full Framework.
       $(RoslynPackageDescriptionDetails)
     </PackageDescription>
     <TargetsForTfmSpecificContentInPackage>$(TargetsForTfmSpecificContentInPackage);_GetFilesToPackage</TargetsForTfmSpecificContentInPackage>