Document System.Threading.Timer.ActiveCount (#2858)

* Document System.Threading.Timer.ActiveCount

* Update Timer.xml

Author: carlossanlop

xml/System.Threading/Timer.xml

@@ -460,8 +460,8 @@ Sub TimerCallback(state As Object)
         <ReturnType>System.Int64</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
+        <summary>Gets the number of timers that are currently active. An active timer is registered to tick at some point in the future, and has not yet been canceled.</summary>
+        <value>The number of timers that are currently active.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -928,4 +928,4 @@ Sub TimerCallback(state As Object)
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>