Update gn's depfile help text to include more guidance.

build/android/gyp/util/build_utils.py

 # Copyright 2013 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.
 
 import ast
 import contextlib
 import fnmatch
 import json
 import os
 import pipes
@@ skipping 402 matching lines @@
   return sorted(set(non_system_module_paths))
 
 
 def AddDepfileOption(parser):
   # TODO(agrieve): Get rid of this once we've moved to argparse.
   if hasattr(parser, 'add_option'):
     func = parser.add_option
   else:
     func = parser.add_argument
   func('--depfile',
-       help='Path to depfile. Must be specified as the action\'s first output.')
+       help='Path to depfile (refer to `gn help depfile`)')
 
 
 def WriteDepfile(depfile_path, first_gn_output, inputs=None, add_pydeps=True):
   assert depfile_path != first_gn_output  # http://crbug.com/646165
   inputs = inputs or []
   if add_pydeps:
     inputs = GetPythonDependencies() + inputs
   MakeDirectory(os.path.dirname(depfile_path))
   # Ninja does not support multiple outputs in depfiles.
   with open(depfile_path, 'w') as depfile:
@@ skipping 102 matching lines @@
 
   md5_check.CallAndRecordIfStale(
       on_stale_md5,
       record_path=record_path,
       input_paths=input_paths,
       input_strings=input_strings,
       output_paths=output_paths,
       force=force,
       pass_changes=True)