Add more verbose description of net_watcher tool.

net/tools/net_watcher/net_watcher.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This is a small utility that watches for and logs network changes.
+// It prints out the current network connection type and proxy configuration
+// upon startup and then prints out changes as they happen.
+// It's useful for testing NetworkChangeNotifier and ProxyConfigService.
+// The only command line option supported is --ignore-netif which is followed
+// by a comma seperated list of network interfaces to ignore when computing
+// connection type; this option is only supported on linux.
 
 #include <memory>
 #include <string>
 #include <unordered_set>
 
 #include "base/at_exit.h"
 #include "base/command_line.h"
 #include "base/compiler_specific.h"
 #include "base/json/json_writer.h"
 #include "base/logging.h"
@@ skipping 203 matching lines @@
   proxy_config_service->RemoveObserver(&net_watcher);
 
   // Uses |network_change_notifier|.
   net::NetworkChangeNotifier::RemoveDNSObserver(&net_watcher);
   net::NetworkChangeNotifier::RemoveConnectionTypeObserver(&net_watcher);
   net::NetworkChangeNotifier::RemoveIPAddressObserver(&net_watcher);
   net::NetworkChangeNotifier::RemoveNetworkChangeObserver(&net_watcher);
 
   return 0;
 }