more description for the look back delta

Signed-off-by: Krasi Georgiev <8903888+krasi-georgiev@users.noreply.github.com>

Author: krasi-georgiev

cmd/thanos/query.go

@@ -70,7 +70,7 @@ func registerQuery(m map[string]setupFunc, app *kingpin.Application) {
 	maxConcurrentQueries := cmd.Flag("query.max-concurrent", "Maximum number of queries processed concurrently by query node.").
 		Default("20").Int()
 
-	lookbackDelta := cmd.Flag("query.lookback-delta", "The maximum lookback duration for retrieving metrics during expression evaluations. Should be set to at least 2 times the slowest scrape interval to avoid gaps in the metrics.").Duration()
+	lookbackDelta := cmd.Flag("query.lookback-delta", "The maximum lookback duration for retrieving metrics during expression evaluations. For example with a look back of 7min and a scrape interval of 10min, promql will fill in the gap only for 7min and will show the other 3min as a gap. Should be set to at least 2 times the slowest scrape interval.").Duration()
 
 	maxConcurrentSelects := cmd.Flag("query.max-concurrent-select", "Maximum number of select requests made concurrently per a query.").
 		Default("4").Int()

docs/components/query.md

@@ -369,7 +369,12 @@ Flags:
                                  concurrently by query node.
       --query.lookback-delta=QUERY.LOOKBACK-DELTA
                                  The maximum lookback duration for retrieving
-                                 metrics during expression evaluations.
+                                 metrics during expression evaluations. For
+                                 example with a look back of 7min and a scrape
+                                 interval of 10min, promql will fill in the gap
+                                 only for 7min and will show the other 3min as a
+                                 gap. Should be set to at least 2 times the
+                                 slowest scrape interval.
       --query.max-concurrent-select=4
                                  Maximum number of select requests made
                                  concurrently per a query.