Add description for compiler packages that are pre-release (#22777)

src/NuGet/BuildNuGets.csx

@@ -143,17 +143,31 @@ string[] TestPackageNames = {
 // or they will not be published anywhere at all
 var PreReleaseOnlyPackages = new HashSet<string>
 {
+    // Contains the compiler build task, but Microsoft.NET.Compilers
+    // should be preferred. Used to construct a standalone compiler for
+    // dotnet CLI.
     "Microsoft.CodeAnalysis.Build.Tasks",
+    
     "Microsoft.CodeAnalysis.CSharp.CodeStyle",
     "Microsoft.CodeAnalysis.VisualBasic.CodeStyle",
     "Microsoft.CodeAnalysis.VisualBasic.Scripting",
+    
+    // Only contains exes. Only used for insertion into dotnet CLI
     "Microsoft.Net.Compilers.netcore",
+    
+    // C# Interactive on CoreCLR is pre-release
     "Microsoft.Net.CSharp.Interactive.netcore",
+    
+    // Pre-release package, API and structure not finalized
     "Microsoft.NETCore.Compilers",
+    
     "Microsoft.CodeAnalysis.Remote.Razor.ServiceHub",
     "Microsoft.CodeAnalysis.Remote.ServiceHub",
     "Microsoft.CodeAnalysis.Remote.Workspaces",
+    
+    // Only used in Roslyn testing code
     "Microsoft.CodeAnalysis.Test.Resources.Proprietary",
+    
     "Microsoft.VisualStudio.IntegrationTest.Utilities",
     "Microsoft.VisualStudio.LanguageServices.Razor.RemoteClient",
     "Microsoft.CodeAnalysis.PooledObjects",