zbus: Fix documentation of ZBUS_LISTENER_DEFINE

The macro is clearly creating the listener in the enabled
state since it is calling the ZBUS_LISTENER_DEFINE_WITH_ENABLE
with 2nd argument being true. Fix the documentation, and add
the same remark to ZBUS_SUBSCRIBER_DEFINE documentation as
well.

Signed-off-by: Miika Karanki <miika.karanki@vaisala.com>