Documented isActive() (#3704)

Documented the help method to provide support for the edge case of detecting the currently active Timeline from a collection of Timelines.

docs/timeline/index.html

@@ -1387,6 +1387,12 @@ document.getElementById('myTimeline').onclick = function (event) {
       <td>Get the current visible window. Returns an object with properties <code>start: Date</code> and <code>end: Date</code>.</td>
     </tr>
 
+    <tr>
+      <td>isActive()</td>
+      <td>Boolean</td>
+      <td>Denotes whether the timeline is currently active. This method only returns <code>false</code> if the timline has the <code>{clickToUse:true}</code> option enabled and the timeline is currently unselected.</td>
+    </tr>
+	  
     <tr>
       <td>moveTo(time [, options, callback])</td>
       <td>none</td>