Document that the activate event is not fired on creation

From http://bugs.jqueryui.com/ticket/8735 & http://bugs.jqueryui.com/ticket/9450.

The current behavior confuses users and should be documented in some obvious manner.

Accordion: http://api.jqueryui.com/accordion/#event-activate
Tabs: http://api.jqueryui.com/tabs/#event-activate
