Description modification for zesDriverEventListen

[bellekal]

I see that pNumDeviceEvents is mentioned as input and output parameter which i believe based on the description says that it's an output parameter. Also, It should be mentioned clearly that at a time, Only one event can be retrieved instead of mentioning atleast as big as count (zes_event_type_flags_t *pEvents), As Event listen api should return as soon as event occurs.