sync: add comment to SyncerProtoUtil

sync/engine/syncer_proto_util.h

Index: sync/engine/syncer_proto_util.h
diff --git a/sync/engine/syncer_proto_util.h b/sync/engine/syncer_proto_util.h
index 8256dbd82c379e17df65af682f99d21da372ffa8..b78a3507c65161a8135be5bfa9b31d6346b41cf6 100644
--- a/sync/engine/syncer_proto_util.h
+++ b/sync/engine/syncer_proto_util.h
@@ -42,6 +42,8 @@ class SyncerProtoUtil {
   // Posts the given message and fills the buffer with the returned value.
   // Returns true on success.  Also handles store birthday verification: will
   // produce a SyncError if the birthday is incorrect.
+  // NOTE: This will add all fields that must be sent on every request, which
+  // includes store birthday, protocol version, client chips, api keys, etc.
   static SyncerError PostClientToServerMessage(
       sync_pb::ClientToServerMessage* msg,
       sync_pb::ClientToServerResponse* response,