spec: update the description of the FlowSegment.timerange

Be specific about when the timerange uses an exclusive (samples have
duration) or inclusive end (samples don't have a duration).

api/schemas/flow-segment.json

@@ -16,7 +16,7 @@
       "$ref": "timestamp.json"
     },
     "timerange": {
-      "description": "The timerange from the first sample in the segment to the last, with an inclusive start and inclusive or exclusive end, as described by the [TimeRange](../schemas/timerange#top) type. Note that where temporal re-ordering is used, this refers to the presentation timeline.",
+      "description": "The timerange for the samples contained in the segment. If samples have a duration then the timerange end is exclusive and covers at least the duration of the last sample. The exclusive timerange end will typically be set to the timestamp of the next sample. If the samples don't have a duration then the timerange end is inclusive. Format is described by the [TimeRange](../schemas/timerange#top) type. Note that where temporal re-ordering is used, the timerange and samples refers to the presentation timeline.",
       "$ref": "timerange.json"
     },
     "last_duration": {