add comment for NewInstance override

src/compiler/csharp_generator.cc

@@ -494,6 +494,9 @@ void GenerateClientStub(Printer *out, const ServiceDescriptor *service) {
   }
 
   // override NewInstance method
+  out->Print(
+      "/// <summary>Creates a new instance of client from given "
+      "<c>ClientBaseConfiguration</c>.</summary>\n");
   out->Print(
       "protected override $name$ NewInstance(ClientBaseConfiguration "
       "configuration)\n",