docs: improve event-related documentation

docs: improve event-related documentation

While looking at event code, I noticed that the documentation was
trying to refer me to functions that don't exist.  Also fix some
typos and poor formatting.

* src/libvirt.c (virConnectDomainEventDeregister)
(virConnectDomainEventRegisterAny)
(virConnectDomainEventDeregisterAny)
(virConnectNetworkEventRegisterAny)
(virConnectNetworkEventDeregisterAny): Link to correct function.
* include/libvirt.h.in (VIR_DOMAIN_EVENT_CALLBACK)
(VIR_NETWORK_EVENT_CALLBACK): Likewise.
(virDomainEventID, virConnectDomainEventGenericCallback)
(virNetworkEventID, virConnectNetworkEventGenericCallback):
Improve docs.

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