JSDoc: move @event to beginning of module and add @fires

Purposes:
* Move @event to the beginning of module, in order to:
  * Stay closer with the way http://usejsdoc.org/tags-event.html specifies how to document events for AMD modules
  * Reduce the risk of declaring the same event more than once.
  * Have a systematic, consistent manner to declare events for events that are fired at one place or at multiple places.
  * Avoid using a jdsoc comment at a line of code which is not a declaration of a class member.
* Add @fires, which brings additional information into the doc.

See also https://github.com/ibm-js/jsdoc-amddcl/issues/41.

The section about events ("Declaring events") has been updated accordingly in 
https://docs.google.com/document/d/1fGBLuDAJRHFuSE4_NM2YfbRcCeYV40KWDXCuVyiSV_U/edit#heading=h.psrcktme7i7o