Initial module docstring

src/compiler/python_generator.cc

@@ -777,7 +777,8 @@ pair<bool, grpc::string> PrivateGenerator::GetGrpcServices() {
     if (generate_in_pb2_grpc) {
       out->Print(
           "# Generated by the gRPC Python protocol compiler plugin. "
-          "DO NOT EDIT!\n");
+          "DO NOT EDIT!\n\"\"\""
+          "Protocol buffer generated client and server classes.\"\"\"\n");
       if (!PrintPreamble(out.get())) {
         return make_pair(false, "");
       }