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

The purpose is to stay closer with the way http://usejsdoc.org/tags-event.html specifies how to document events for AMD modules, and to reduce the risk of declaring the same event more than once.

See also https://github.com/ibm-js/jsdoc-amddcl/issues/41.

The section about events has been updated accordingly in https://docs.google.com/document/d/1fGBLuDAJRHFuSE4_NM2YfbRcCeYV40KWDXCuVyiSV_U/edit?usp=sharing .