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

[miket_OOO, 2013/10/15 18:17:33]

Because this is documentation, we should be more careful about style. "URL" is
capitalized.

[bshe, 2013/10/16 20:28:30]

Done.

+permission.

[miket_OOO, 2013/10/15 18:17:33]

"in the manifest permissions."

[bshe, 2013/10/16 20:28:30]

Done.

+For example:</p>
+<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>
+
+<pre>
+chrome.wallpaper.setWallpaper(
+    {
+      'url': 'http://example.com/aFile.jpg',

[miket_OOO, 2013/10/15 18:17:33]

This is horribly exacting, but our Chrome style guidelines likely wouldn't have
permitted this file name. I know it's just a sample, but a_file.jpg is more
consistent.

[bshe, 2013/10/16 20:28:30]

Done.

+      'layout': 'CENTER_CROPPED',
+      'name': 'test_wallpaper'
+    }, function() { console.error(chrome.runtime.lastError.message); });
+</pre>