Improve command-line help messages in debugger.

(also rename some methods and move them around)

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1181663003.

mojo/devtools/common/debugger

diff --git a/mojo/devtools/common/debugger b/mojo/devtools/common/debugger
index 446923c..4ff747f 100755
--- a/mojo/devtools/common/debugger
+++ b/mojo/devtools/common/debugger
@@ -16,31 +16,13 @@
   return requests.get(url)
 
 
-def add_tracing_command(subparsers):
-  """Sets up the command line parser to manage tracing."""
-  tracing_parser = subparsers.add_parser(
-      'tracing', help='Tracing commands for mojo:debugger')
-  tracing_subparser = tracing_parser.add_subparsers(
-      help='tracing sub-command help')
-
-  start_tracing_parser = tracing_subparser.add_parser('start',
-                                                      help='starts tracing')
-  start_tracing_parser.set_defaults(func=start_tracing_command)
-
-  stop_tracing_parser = tracing_subparser.add_parser('stop',
-                                                     help='stops tracing')
-  stop_tracing_parser.add_argument('file_name', type=str,
-                                   default='mojo.trace')
-  stop_tracing_parser.set_defaults(func=stop_tracing_command)
-
-
-def start_tracing_command(_):
+def _tracing_start(_):
   """Starts tracing."""
   _send_request('start_tracing')
   print "Started tracing."
 
 
-def stop_tracing_command(args):
+def _tracing_stop(args):
   """Stops tracing and writes trace to file."""
   file_name = args.file_name
   trace = _send_request('stop_tracing').content
@@ -51,12 +33,29 @@
   print "Trace saved in %s" % file_name
 
 
+def _add_tracing_command(subparsers):
+  """Sets up the command line parser to manage tracing."""
+  tracing_parser = subparsers.add_parser('tracing',
+      help='trace event profiler')
+  tracing_subparser = tracing_parser.add_subparsers(
+      help='the command to run')
+
+  start_tracing_parser = tracing_subparser.add_parser('start',
+      help='start tracing')
+  start_tracing_parser.set_defaults(func=_tracing_start)
+
+  stop_tracing_parser = tracing_subparser.add_parser('stop',
+      help='stop tracing and retrieve the result')
+  stop_tracing_parser.add_argument('file_name', type=str,
+                                   default='mojo.trace')
+  stop_tracing_parser.set_defaults(func=_tracing_stop)
+
+
 def main():
   parser = argparse.ArgumentParser(description='Command-line interface for '
                                                 'mojo:debugger')
-  subparsers = parser.add_subparsers(help='sub-command help')
-  add_tracing_command(subparsers)
-
+  subparsers = parser.add_subparsers(help='the tool to run')
+  _add_tracing_command(subparsers)
 
   args = parser.parse_args()
   args.func(args)