Improve warning about deprecation in PRESUBMIT

BUG=None
NOTRY=True

Review-Url: https://codereview.webrtc.org/2691933003
Cr-Commit-Position: refs/heads/master@{#16575}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 6e5fb59..3405ef9 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -117,8 +117,9 @@
 
 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