[Sync] Add comment for SyncBackendHost::OnBackendInitialized

chrome/browser/sync/engine/syncapi.h

 // Copyright (c) 2011 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.
 
 // This file defines the "sync API", an interface to the syncer
 // backend that exposes (1) the core functionality of maintaining a consistent
 // local snapshot of a hierarchical object set; (2) a means to transactionally
 // access and modify those objects; (3) a means to control client/server
 // synchronization tasks, namely: pushing local object modifications to a
 // server, pulling nonlocal object modifications from a server to this client,
@@ skipping 827 matching lines @@
     virtual void OnPassphraseAccepted(const std::string& bootstrap_token) = 0;
 
     // Called when initialization is complete to the point that SyncManager can
     // process changes. This does not necessarily mean authentication succeeded
     // or that the SyncManager is online.
     // IMPORTANT: Creating any type of transaction before receiving this
     // notification is illegal!
     // WARNING: Calling methods on the SyncManager before receiving this
     // message, unless otherwise specified, produces undefined behavior.
     //
-    // The given backend can emit the following events:
+    // |js_backend| is what about:sync interacts with.  It can emit
+    // the following events:
 
     /**
      * @param {{ enabled: boolean }} details A dictionary containing:
      *     - enabled: whether or not notifications are enabled.
      */
     // function onNotificationStateChange(details);
 
     /**
      * @param {{ changedTypes: Array.<string> }} details A dictionary
      *     containing:
      *     - changedTypes: a list of types (as strings) for which there
              are new updates.
      */
     // function onIncomingNotification(details);
 
-    // The given backend responds to the following messages (all other
-    // messages are ignored):
+    // Also, it responds to the following messages (all other messages
+    // are ignored):
 
     /**
      * Gets the current notification state.
      *
      * @param {function(boolean)} callback Called with whether or not
      *     notifications are enabled.
      */
     // function getNotificationState(callback);
 
     /**
@@ skipping 209 matching lines @@
  private:
   // An opaque pointer to the nested private class.
   SyncInternal* data_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncManager);
 };
 
 }  // namespace sync_api
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_SYNCAPI_H_