Clarify how `new-branch` is different for --help

git_new_branch.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
+"""
+Create new branch tracking origin/master by default.
+"""
+
 import argparse
 import sys
 
 import subprocess2
 
 from git_common import run, root, set_config, get_or_create_merge_base, tags
 from git_common import hash_one
 
 
 def main(args):
   parser = argparse.ArgumentParser(
-    formatter_class=argparse.ArgumentDefaultsHelpFormatter
+    formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    description=__doc__,
   )
   parser.add_argument('branch_name')
   g = parser.add_mutually_exclusive_group()
   g.add_argument('--upstream-current', '--upstream_current',
                  action='store_true',
                  help='set upstream branch to current branch.')
   g.add_argument('--upstream', metavar='REF', default=root(),
                  help='upstream branch (or tag) to track.')
   g.add_argument('--lkgr', action='store_const', const='lkgr', dest='upstream',
                  help='set basis ref for new branch to lkgr.')
@@ skipping 22 matching lines @@
   sys.stderr.write('Switched to branch %s.\n' % opts.branch_name)
   return 0
 
 
 if __name__ == '__main__':  # pragma: no cover
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)