Adding recommendation for alert rule lookbehind window when recording rule is in use

docs/victoriametrics/vmalert.md

@@ -972,6 +972,8 @@ Try the following recommendations to reduce the chance of hitting the data delay
   in datasource is inconsistent or `>=5min` - try changing vmalerts `-datasource.queryStep` command-line flag to specify
   how far search query can look back for the recent datapoint. The recommendation is to have the step
   at least two times bigger than the resolution.
+* If alert is based on recording rule, make sure that lookbehind window of alerting rule (see -datasource.queryStep duration) is not matching evaluation interval of a recording rule, otherwise there is a chance that alerting rule occasionally won't see the latest data generated by recording rule, causing "flapping" alerts. If this is the case, the recommendation is to increase lookbehind window to be 2x higher than recording rule evaluation interval
+
 
 > Please note, data delay is inevitable in distributed systems. And it is better to account for it instead of ignoring.