Add docs for chrome.wallpaper API

chrome/common/extensions/docs/templates/json/permissions.json

 {
   "host-permissions": {
     "anchor": "match-pattern",
     "name": "[scheme]:[host]/*",
     "partial": "permissions/host_permissions.html",
     "platforms": ["app", "extension"]
   },
   "activeTab": {
     "partial": "permissions/active_tab.html"
   },
@@ skipping 35 matching lines @@
   },
   "tabs": {
     "partial": "permissions/tabs.html"
   },
   "unlimitedStorage": {
     "partial": "permissions/unlimited_storage.html"
   },
   "videoCapture": {
     "partial": "permissions/video_capture.html"
   },
+  "wallpaper": {
+    "partial": "permissions/wallpaper.html"
+  },
   "webRequestBlocking": {
     "partial": "permissions/web_request_blocking.html"
   },
   "webview": {
     "partial": "permissions/webview.html"
   }
 }