Add sync and PrefStore docs to prefs/README.md

[email protected], [email protected], [email protected]

Follow-up to:
https://chromium-review.googlesource.com/c/chromium/src/+/2812102/2..4/chrome/browser/prefs/README.md#b49
and
https://chromium-review.googlesource.com/c/chromium/src/+/2808217/4..5/chrome/browser/prefs/browser_prefs.cc#b654

Change-Id: I60d5ac4b17821b0a6c58a58e75cef5ae51999f0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2815359
Reviewed-by: Julian Pastarmov <[email protected]>
Reviewed-by: Dominic Battré <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Reviewed-by: Jan Krcal <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#871926}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 74da069..6e4453d 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -5408,7 +5408,8 @@
       'Please make sure to properly deprecate preferences by clearing their\n'
       'value for a couple of milestones before finally removing the code.\n'
       'Otherwise data may stay in the preferences files forever. See\n'
-      'Migrate*Prefs() in chrome/browser/prefs/browser_prefs.cc for examples.\n'
+      'Migrate*Prefs() in chrome/browser/prefs/browser_prefs.cc and\n'
+      'chrome/browser/prefs/README.md for examples.\n'
       'This may be a false positive warning (e.g. if you move preference\n'
       'registrations to a different place).\n',
       potential_problems