Fix the documentation in sessions API.

Add info for when sessionId is not provided in the restore function.

R=kalman@chromium.org
BUG=377607

Review URL: https://codereview.chromium.org/295423005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273028 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/sessions.json

@@ -91,7 +91,7 @@
             "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",