Fix the documentation in sessions API.

chrome/common/extensions/api/sessions.json

 // Copyright 2013 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.
 
 [
   {
     "namespace": "sessions",
     "description": "Use the <code>chrome.sessions</code> API to query and restore tabs and windows from a browsing session.",
     "types": [
       {
@@ skipping 72 matching lines @@
       },
       {
         "name": "restore",
         "type": "function",
         "description": "Reopens a $(ref:windows.Window) or $(ref:tabs.Tab), with an optional callback to run when the entry has been restored.",
         "parameters": [
           {
             "type": "string",
             "name": "sessionId",
             "optional": true,
-            "description": "The $(ref:windows.Window.sessionId), or $(ref:tabs.Tab.sessionId) to restore."
+            "description": "The $(ref:windows.Window.sessionId), or $(ref:tabs.Tab.sessionId) to restore. If this parameter is not specified, the most recently closed session is restored."
           },
           {
             "type": "function",
             "name": "callback",
             "optional": true,
             "parameters": [
               {
                 "$ref": "Session",
                 "name": "restoredSession",
                 "description": "A $(ref:sessions.Session) containing the restored $(ref:windows.Window) or $(ref:tabs.Tab) object."
@@ skipping 11 matching lines @@
       }
     ],
     "properties": {
       "MAX_SESSION_RESULTS": {
         "value": 25,
         "description": "The maximum number of $(ref:sessions.Session) that will be included in a requested list."
       }
     }
   }
 ]