Add info about "background" permission (new for M10).

chrome/common/extensions/docs/static/background_pages.html

 <div id="pageData-name" class="pageData">Background Pages</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 A common need for extensions is to have
 a single long-running script to manage some task or state.
 Background pages to the rescue.
 </p>
 
 <p>
@@ skipping 23 matching lines @@
 like this:
 </p>
 
 <pre>{
   "name": "My extension",
   ...
   <b>"background_page": "background.html"</b>,
   ...
 }</pre>
 
+<p>
+If you need the browser to start up early&mdash;so
+you can display notifications, for example&mdash;then
+you might also want to specify the
+<a href="manifest.html#permissions">"background" permission</a>.
+</p>
+
+
 <h2>Details</h2>
 
 <p>
 You can communicate between your various pages using direct script calls,
 similar to how frames can communicate.
 The <a href="extension.html#method-getViews"><code>chrome.extension.getViews()</code></a> method
 returns a list of window objects
 for every active page belonging to your extension,
 and the
 <a href="extension.html#method-getBackgroundPage"><code>chrome.extension.getBackgroundPage()</code></a> method
@@ skipping 60 matching lines @@
     &lt;p>
     Image here:
     &lt;/p>
 
     &lt;img id="target" src="white.png" width="640" height="480">
 
   &lt;/body>
 &lt;/html>
 </pre>