doc: event notifier on kretprobe is not supported

doc: event notifier on kretprobe is not supported

The kretprobe behavior is to fire twice (entry and exit of target
function), placing an event notifier on such function does not make
sense at first glance. If we come up with a use case it will be quite
easy to enable.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I6f214501706d1ef170c81b80a1f82c039d687502