doc: announce changes to eventdev library

doc: announce changes to eventdev library

Make driver layer as internal, remove unnecessary rte_ prefix for
structures and functions that are not a part of public API.
Promote experimental trace and vector APIs to stable.
Add reserved field to `rte_event_timer` structure.

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
Acked-by: Jay Jayatheerthan <jay.jayatheerthan@intel.com>
Acked-by: Abhinandan Gujjar <abhinandan.gujjar@intel.com>