debugger: improve help information

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: https://github.com/joyent/node/pull/14172

lib/_debugger.js

@@ -33,6 +33,8 @@ exports.start = function(argv, stdin, stdout) {
 
   if (argv.length < 1) {
     console.error('Usage: node debug script.js');
+    console.error('       node debug <host>:<port>');
+    console.error('       node debug -p <pid>');
     process.exit(1);
   }