updated event_index and event_id description

base_classes/NXevent_data.nxdl.xml

@@ -60,7 +60,9 @@
 	<field name="event_id" type="NX_INT" units="NX_DIMENSIONLESS">
 		<doc>
 			There will be extra information in the NXdetector to convert 
-			event_id to detector_number.
+			event_id to detector_number. But in short, 
+			event_id contains information about detector_id which is 
+			related to the specific pixels on the detector bank.
 		</doc>
 		<dimensions rank="1"><dim index="1" value="i"/></dimensions>
 	</field>
@@ -77,6 +79,10 @@
 	  <doc>
 	    The index into the event_time_offset, event_id pair for
 	    the pulse occurring at the matching entry in event_time_zero.
+	    Each pulse creates an entry in the event_index, 
+	    the value of event_index points to the latest index of the event in event_id. 
+	    Sometimes, two adjcent event_index[i] and event_index[i+1] will have the same value, 
+	    this means there is no additonal neutron hit the detector bank in (i+1)th pulse.
 	  </doc>
 		<dimensions rank="1"><dim index="1" value="j"/></dimensions>
 	</field>