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 specify

[not at google - send to devlin, 2013/10/18 00:18:38]

"you must declare..." can be left out, the intro table should cover it.

"if you use the url property..." well firstly why does that require host
permissions? And secondly, you can put that text in intro_tables.json.

+<a href="declare_permissions.html">host permissions</a> for the URL in the
+manifest.
+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>
+For example, to set the wallpaper as the image at
+<code>http://example.com/a_file.png</code>, you can call
+<code>chrome.wallpaper.setWallpaper</code> this way:
+</p>
+
+<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>