Update configuration docs

docs/sources/img/installation-datasource_config.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:2a0416ac20486db595be1139671679fc7177e59644f0131027f4843bfa806d71
-size 141448
+oid sha256:6b58742637f58519a5fab038db0c15b5525d1d330fe5e4a1de954b72f4f1cc3e
+size 225446

docs/sources/installation/configuration.md

@@ -40,14 +40,24 @@ support in Zabbix 2.x ([ZBXNEXT-1193](https://support.zabbix.com/browse/ZBXNEXT-
     strictly recommended for displaying long time periods (more than few days, depending of your item's
     updating interval in Zabbix) because few days of item history contains tons of points. Using trends
     will increase Grafana performance.
-    - **Use trends from**: time after which trends will be used. Default is **7d** (7 days).
+    - **After**: time after which trends will be used. 
+        Best practice is to set this value to your history storage period (7d, 30d, etc). Default is **7d** (7 days).
         You can set the time in Grafana format. Valid time specificators are:
         - **h** - hours
         - **d** - days
         - **M** - months
-- **Cache update interval**: plugin caches some api requests for increasing performance. Set this
+    - **Range**: Time range width after which trends will be used instead of history.
+        It's better to set this value in range of 4 to 7 days to prevent loading large amount of history data.
+        Default is 4 days.
+- **Cache TTL**: plugin caches some api requests for increasing performance. Set this
     value to desired cache lifetime (this option affect data like items list).
 
+### Alerting
+- **Enable alerting**: enable limited alerting support.
+- **Add thresholds**: get thresholds info from zabbix triggers and add it to graphs.
+    For example, if you have trigger `{Zabbix server:system.cpu.util[,iowait].avg(5m)}>20`, threshold will be set to 20.
+- **Min severity**: minimum trigger severity for showing alert info (OK/Problem).
+
 Then click _Add_ - datasource will be added and you can check connection using 
 _Test Connection_ button. This feature can help to find some mistakes like invalid user name 
 or password, wrong api url.