Add the missing comment for main method

Author: Sean0628

Diff for: content/docs/tutorials/basic/ruby.md

@@ -273,6 +273,9 @@ s = GRPC::RpcServer.new
 s.add_http2_port(port, :this_port_is_insecure)
 GRPC.logger.info("... running insecurely on #{port}")
 s.handle(ServerImpl.new(feature_db))
+# Runs the server with SIGHUP, SIGINT and SIGQUIT signal handlers to
+#   gracefully shutdown.
+# User could also choose to run server via call to run_till_terminated
 s.run_till_terminated_or_interrupted([1, 'int', 'SIGQUIT'])
 ```
 As you can see, we build and start our server using a `GRPC::RpcServer`. To do