protocol: Add summaries to event parameters

All event arg elements now have an appropriate summary attribute.
This was conducted mostly in response to the undocumented parameter
warnings generated during 'make check'.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>