Add documentation for filtered events.

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

 <p>
 An <code>Event</code> is an object
 that allows you to be notified
 when something interesting happens.
 Here's an example of using the
 <code>chrome.tabs.onCreated</code> event
 to be notified whenever there's a new tab:
 </p>
 
 <pre>
@@ skipping 136 matching lines @@
 <pre>
 var rule_ids = ["id1", "id2", ...];
 function getRules(rule_ids, function callback(details) {...});
 </pre>
 
 <p>
 The <code>details</code> parameter passed to the <code>calback()</code> function
 refers to an array of rules including filled optional parameters.
 </p>
 </div>
+
+<div class="doc-family extensions">
+<h2 id="filtered">Filtered events</h2>
+
+<p>Filtered events are a mechanism that allows listeners to specify a subset of
+events that they are interested in. A listener that makes use of a filter won't
+be invoked for events that don't pass the filter, which makes the listening
+code more declarative and efficient - an <a href="event_pages.html">event
+  page</a> page need not be woken up to handle events it doesn't care
+about.</p>
+
+<p>Filtered events are intended to allow a transition from manual filtering
+code like this:</p>
+
+<pre>
+chrome.webNavigation.onCommitted.addListener(function(e) {
+  if (hasHostSuffix(e.url, 'google.com') ||
+      hasHostSuffix(e.url, 'google.com.au')) {
+    // ...
+  }
+});
+</pre>
+
+<p>into this:</p>
+
+<pre>
+chrome.webNavigation.onCommitted.addListener(function(e) {
+  // ...
+}, {url: [{hostSuffix: 'google.com'},
+          {hostSuffix: 'google.com.au'}]});
+</pre>
+
+<p>Events support specific filters that are meaningful to that event. The list
+of filters that an event supports will be listed in the documentation for that
+event in the "filters" section.</p>
+
+<p>When matching URLs (as in the example above), event filters support the same
+URL matching capabilities as expressable with a <a
+  href="events.html#type-UrlFilter">UrlFilter</a>, except for scheme and port
+matching.</p>
+
+</div>