update wiki pages

Single-server-VictoriaMetrics.md

@@ -1113,7 +1113,8 @@ A rough estimation of the required resources for ingestion path:
 
 * Storage space: less than a byte per data point on average. So, ~260GB is required for storing a month-long insert stream
   of 100K data points per second.
-  The actual storage size heavily depends on data randomness (entropy). Higher randomness means higher storage size requirements.
+  The actual storage size heavily depends on data randomness (entropy) and the average number of samples per time series.
+  Higher randomness means higher storage size requirements. Lower average number of samples per time series means higher storage requirement.
   Read [this article](https://medium.com/faun/victoriametrics-achieving-better-compression-for-time-series-data-than-gorilla-317bc1f95932)
   for details.
 

vmalert.md

@@ -89,6 +89,12 @@ name: <string>
 # By default "prometheus" rule type is used.
 [ type: <string> ]
 
+# Optional list of label filters applied to every rule's
+# request withing a group. Is compatible only with VM datasource.
+# See more details at https://docs.victoriametrics.com#prometheus-querying-api-enhancements
+extra_filter_labels:
+  [ <labelname>: <labelvalue> ... ]
+
 rules:
   [ - <rule> ... ]
 ```