Add more verbose description of net_watcher tool.

net/tools/net_watcher/net_watcher.cc

Index: net/tools/net_watcher/net_watcher.cc
diff --git a/net/tools/net_watcher/net_watcher.cc b/net/tools/net_watcher/net_watcher.cc
index b623954c6e6a497c92224c5b8af412125900c5da..49a7d04f3f236b79c05b766d67b9683233c8734b 100644
--- a/net/tools/net_watcher/net_watcher.cc
+++ b/net/tools/net_watcher/net_watcher.cc
@@ -3,6 +3,12 @@
 // 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>