Improve warning about deprecation in PRESUBMIT

PRESUBMIT.py

 # Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import json
 import os
@@ skipping 99 matching lines @@
   if non_existing_paths:
     return [output_api.PresubmitError(
         'Directories to native API headers have changed which has made the '
         'list in PRESUBMIT.py outdated.\nPlease update it to the current '
         'location of our native APIs.',
         non_existing_paths)]
   return []
 
 api_change_msg = """
 You seem to be changing native API header files. Please make sure that you:
-  1. Make compatible changes that don't break existing clients.
-  2. Mark the old stuff as deprecated.
+  1. Make compatible changes that don't break existing clients. Usually
+     this is done by keeping the existing method signatures unchanged.
+  2. Mark the old stuff as deprecated (see RTC_DEPRECATED macro).
   3. Create a timeline and plan for when the deprecated stuff will be
      removed. (The amount of time we give users to change their code
      should be informed by how much work it is for them. If they just
      need to replace one name with another or something equally
      simple, 1-2 weeks might be good; if they need to do serious work,
      up to 3 months may be called for.)
   4. Update/inform existing downstream code owners to stop using the
      deprecated stuff. (Send announcements to
      discuss-webrtc@googlegroups.com and webrtc-users@google.com.)
   5. Remove the deprecated stuff, once the agreed-upon amount of time
@@ skipping 406 matching lines @@
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   results.extend(_CheckChangeHasBugField(input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasTestField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckTreeIsOpen(
       input_api, output_api,
       json_url='http://webrtc-status.appspot.com/current?format=json'))
   return results