Add docs for chrome.wallpaper API

chrome/common/extensions/docs/templates/intros/wallpaper.html

+<h2 id="manifest">Manifest</h2>
+<p>You must declare the "wallpaper" permission in the app
+<a href="manifest.html">manifest</a> to use the wallpaper API. If you use the
+url property of setWallpaper, you must also whitelist the domain of your URL in
+the manifest permissions.
+For example:</p>

[not at google - send to devlin, 2013/10/16 21:17:27]

This text should go in permissions.json:

https://code.google.com/p/chromium/codesearch#chromium/src/chrome/common/exte...

see pattern there.

as discussed you need to
- un-nodoc wallpaper.setWallpaper
- add a description to the schema.

[bshe, 2013/10/17 18:13:32]

Done.

+<pre data-filename="manifest.json">
+{
+  "name": "My extension",
+  ...
+  <b>"permissions": [
+    "wallpaper",
+    "http://example.com/*"
+  ]</b>,
+  ...
+}</pre>
+
+<h2 id="overview-examples">Examples</h2>
+
+<p>
+Here's sample code to change wallpaper:
+</p>

[not at google - send to devlin, 2013/10/16 21:17:27]

This paragraph isn't much use. You can make it better:

"For example, to set the wallpaper as the image at
<code>http://example.com/a_file.png", .....

[bshe, 2013/10/17 18:13:32]

Done.

+
+<pre>
+chrome.wallpaper.setWallpaper(
+    {
+      'url': 'http://example.com/a_file.jpg',
+      'layout': 'CENTER_CROPPED',
+      'name': 'test_wallpaper'
+    }, function() { console.error(chrome.runtime.lastError.message); });
+</pre>