Add docs for chrome.wallpaper API

chrome/common/extensions/api/wallpaper.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":"wallpaper",
     "compiler_options": {
       "implemented_in": "chrome/browser/chromeos/extensions/wallpaper_api.h"
     },
     "platforms": ["chromeos"],
-    "description": "none",
+    "description": "Use the <code>chrome.wallpaper</code> API to change the ChromeOS wallpaper.",
     "functions": [
     {
       "name": "setWallpaper",
       "type": "function",
-      "description": "Sets wallpaper to the image from url with specified layout",
-      "nodoc": "true",
+      "description": "Sets wallpaper to the image at <code>url</code> or <code>wallpaperData</code> with the specified layout",
       "parameters": [
         {
           "name": "details",
           "type": "object",
           "properties": {
             "wallpaperData": {
               "type": "binary",
               "optional": true,
               "description": "The jpeg or png encoded wallpaper image."
             },
             "url": {
               "type": "string",
               "optional": true,
-              "description": "The url of online wallpaper."
+              "description": "The URL of the wallpaper to be set. If host permissions for this URL are not specified in the manifest file, the API call will fail."
             },
             "layout": {
               "type": "string",
               "enum": [ "STRETCH", "CENTER", "CENTER_CROPPED" ],
               "description": "The supported wallpaper layouts."
             },
             "name": {
               "type": "string",
-              "description": "The file name of saved wallpaper."
+              "description": "The file name of the saved wallpaper."
             },
             "thumbnail": {
               "type": "boolean",
               "optional": true,
               "description": "True if a 128x60 thumbnail should be generated."
             }
           }
         },
         {
           "type": "function",
           "name": "callback",
           "parameters": [
             {
               "type": "binary",
               "optional": true,
-              "name": "thumbnail"
+              "name": "thumbnail",
+              "description": "The jpeg encoded wallpaper thumbnail. It is generated by resizing the wallpaper to 128x60."
             }
           ]
         }
       ]
     }
     ]
   }
 ]