Improve IFEX Core IDL documentation in the section regarding Events

From a review of docs update from Erik Jaegervall #60.
Improve the section that has confusing asterisks / comments / notes.

Start by reviewing the Events section of the IFEX Core IDL documentation and the documentation update referenced in issue #60. Clarify the confusing asterisks, comments, and notes, then confirm the revised section is understandable and consistent with the surrounding documentation.

Written by the indexing model from the issue text.