event: fix doc typos, and doc more public methods

event: fix doc typos, and doc more public methods

While working on events, I found a number of minor issues; I'm
hoisting these to the front rather than doing it piecemeal in
the patches where I first noticed bad or missing documentation.

* src/conf/object_event.c: Fix grammar, document all parameters
of public functions, wrap some long lines.
* src/conf/object_event.h: Likewise.
* src/conf/network_event.c: Likewise.
* src/conf/domain_event.c: Likewise (except for the large number
of event creation functions).
* src/libvirt_private.cyms (conf/object_event.h): Split...
(conf/network_event.h): ...to account for new file.

Signed-off-by: Eric Blake <eblake@redhat.com>