Improve docs

docs/uptime/overview.asciidoc

@@ -39,7 +39,9 @@ selected date range.
 The counts for this view reflect data in realtime except in certain cases where computing
 the counts efficiently in an exact manner is not possible. This only happens when there are 
 more than 1000 monitors up and down at the same time. In these cases results will be delayed
-by up to 30 seconds.
+by up to 30 seconds. If data is not searchable in less than 30 seconds the snapshot widget
+will show no data. This can happen if one uses many intermediate queues between heartbeat and
+Elasticsearch, or if high index refresh settings are enabled in Elasticsearch.
 
 [float]
 === Monitor list