MINOR: event_hdl: global sublist management clarification

MINOR: event_hdl: global sublist management clarification

event_hdl_sub_list_init() and event_hdl_sub_list_destroy() don't expect
to be called with a NULL argument (to use global subscription list
implicitly), simply because the global subscription list init and
destroy is internally managed.

Adding BUG_ON() to detect such invalid usages, and updating some comments
to prevent confusion around these functions.

If 68e692da0 ("MINOR: event_hdl: add event handler base api")
is being backported, then this commit should be backported with it.