Add changelog and mention behaviour in documentation

Signed-off-by: Kemal Akkoyun <[email protected]>

CHANGELOG.md

@@ -34,6 +34,7 @@ We use *breaking* word for marking changes that are not backward compatible (rel
 ### Added
 
 - [#2671](https://github.com/thanos-io/thanos/pull/2671) Tools: bucket replicate now allows passing repeated `--compaction` and `--resolution` flags.
+- [#2657](https://github.com/thanos-io/thanos/pull/2657) Querier: Now, has the ability to perform concurrent select request per query.
 
 ## [v0.13.0](https://github.com/thanos-io/thanos/releases) - IN PROGRESS
 
@@ -819,4 +820,4 @@ Initial version to have a stable reference before [gossip protocol removal](/doc
 - Compact / Downsample offline commands.
 - Bucket commands.
 - Downsampling support for UI.
-- Grafana dashboards for Thanos components.
+- Grafana dashboards for Thanos components.

docs/components/query.md

@@ -216,6 +216,12 @@ type queryData struct {
 Additional field is `Warnings` that contains every error that occurred that is assumed non critical. `partial_response`
 option controls if storeAPI unavailability is considered critical.
 
+### Concurrent Selects
+
+Thanos Querier has the ability to perform concurrent select request per query. It dissects given PromQL statement and executes selectors concurrently against the discovered StoreAPIs.
+The maximum number of concurrent requests are being made per query is controller by `query.max-concurrent-select` flag.
+Keep in mind that the maximum number of concurrent queries that are handled by querier is controlled by `query.max-concurrent`. Please consider implications of combined value while tuning the querier.
+
 ## Expose UI on a sub-path
 
 It is possible to expose thanos-query UI and optionally API on a sub-path.