Document the limitations on chrome.alarms timing.

chrome/common/extensions/docs/templates/intros/alarms.html

Index: chrome/common/extensions/docs/templates/intros/alarms.html
diff --git a/chrome/common/extensions/docs/templates/intros/alarms.html b/chrome/common/extensions/docs/templates/intros/alarms.html
index f8bcb4ea73d76c3f99911ee84d94ec19fbbec3e7..290074f232c434e274a81eeed7db32b5ad9f0bc0 100644
--- a/chrome/common/extensions/docs/templates/intros/alarms.html
+++ b/chrome/common/extensions/docs/templates/intros/alarms.html
@@ -21,4 +21,19 @@
     <td><strong>Learn more:</strong></td>
     <td><a href="event_pages.html">Event Pages</a></td>
   </tr>
-</table>
+</table>
+
+<h2 id="frequency-limits">Alarm frequency limits</h2>
+<p>In order to reduce the load on the user's machine, Chrome limits
+alarms to at most once every 5 minutes but may delay them an arbitrary
+amount more.  That is, setting
+<code>$ref:[alarms.AlarmCreateInfo.delayInMinutes
+delayInMinutes]</code> or
+<code>$ref:[alarms.AlarmCreateInfo.periodInMinutes
+periodInMinutes]</code> to less than <code>5</code> will cause an
+error.  <code>$ref:[alarms.AlarmCreateInfo.when when]</code> can be
+set to less than 5 minutes after "now" but won't actually cause the
+alarm to run for at least 5 minutes.</p>
+
+<p>To help you debug your app or extension, when you've loaded it
+unpacked, there's no limit to how often the alarm can fire.</p>