updated event_index and event_id description

[KyleQianliMa]

Description

The description of event_index can be confusing for new developers. I added a few lines of description in event_index and event_id to make it more clear.

Details:

Event_index is associated with pulses. Each pulse creates an entry in event_index, the values of event_index points to event_id and event_id contains information about detector_id.

Example:
event_index:
0:0
1:1
2:1
3:1
4:3

Event_id:
0:490
1:231
2:434
3:611

This means at pulse 0, current detector bank detected an event 0, the det_id for this event is event_id[0] = 490. For pulse 1, a new event is detected and indexed 1, the det_id for this event is event_id[1] = 231. For pulse 2 and pulse 3 no events are detected on this detector bank therefore the event_index for these 2 pulses remain 1. For pulse 4, 2 events are detected, the last index of the detected event is 3, meaning event 2 and 3 are created from this pulse and their det_ids are registered as 434 and 611.

This PR added a short paragraph in event_index to illustrate the description above. Also a little bit in event_id to make it easier to understand.

[KedoKudo]

@phyy-nx Could you review this PR? Thanks