Clarify chrome.events as a namespace of common types.

chrome/common/extensions/docs/templates/intros/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.alarms.onAlarm</code> event
 to be notified whenever an alarm has elapsed:
 </p>
 
 <pre>
@@ skipping 11 matching lines @@
 is always a function that you define to handle the event,
 but the parameters to the function depend on
 which event you're handling.
 Checking the documentation for
 $ref:alarms.onAlarm,
 you can see that the function has a single parameter:
 an $ref:alarms.Alarm object
 that has details about the elapsed alarm.
 </p>
 
+Example APIs using Events:
+$ref:alarms,
+{{?is_apps}}
+$ref:app.runtime,
+$ref:app.window,
+{{/is_apps}}
+$ref:i18n,
+$ref:identity,
+$ref:runtime.
+Most <a href="api_index.html">chrome APIs</a> do.
 
 {{^is_apps}}
 <div class="doc-family extensions">
 <h2 id="declarative">Declarative Event Handlers</h2>
 
 <p>
 The declarative event handlers provide a means to define rules consisting of
 declarative conditions and actions. Conditions are evaluated in the browser
 rather than the JavaScript engine which reduces roundtrip latencies and allows
 for very high efficiency.
@@ skipping 212 matching lines @@
 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 expressible with a
   $ref:events.UrlFilter, except for scheme and port
 matching.</p>
 
 </div>
 {{/is_apps}}