ES|QL: Add Known Issues note about STATS (v 8.x) elastic#130644 (elastic#131001)

Author: luigidellaquila

docs/reference/esql/esql-limitations.asciidoc

@@ -302,3 +302,17 @@ the <<esql-mv-functions,multivalue functions>>.
 === Kibana limitations
 
 include::esql-kibana.asciidoc[tag=limitations]
+
+[discrete]
+[[esql-known-issues]]
+== Known issues
+
+A bug in the ES|QL STATS command may yield incorrect results. The bug only happens in very specific cases that follow this pattern: `STATS ... BY keyword1, keyword2`, i.e. the command must have exactly two grouping fields, both keywords, where the first field has high cardinality (more than 65k distinct values).
+
+The bug is described in detail in [this issue](https://github.com/elastic/elasticsearch/issues/130644).
+The problem was introduced in 8.16.0 and [fixed](https://github.com/elastic/elasticsearch/pull/130705) in 8.17.9, 8.18.7.
+
+Possible workarounds include:
+  * switching the order of the grouping keys (eg. `STATS ... BY keyword2, keyword1`, if the `keyword2` has a lower cardinality)
+  * reducing the grouping key cardinality, by filtering out values before STATS
+

docs/reference/release-notes/8.16.0.asciidoc

@@ -517,3 +517,13 @@ Snapshot/Restore::
 {esql}::
 
 * Some valid queries using an `ENRICH` command can fail when a match field is used that is absent from some indices or shards, either with a 500 status code due to `NullPointerException` or `ClassCastException` or with a 400 status code and `IllegalArgumentException`. This is fixed in {es-pull}126187[#126187].
+
+* A bug in the ES|QL STATS command may yield incorrect results. The bug only happens in very specific cases that follow this pattern: `STATS ... BY keyword1, keyword2`, i.e. the command must have exactly two grouping fields, both keywords, where the first field has high cardinality (more than 65k distinct values).
+
+  The bug is described in detail in [this issue](https://github.com/elastic/elasticsearch/issues/130644).
+  The problem was introduced in 8.16.0 and [fixed](https://github.com/elastic/elasticsearch/pull/130705) in 8.17.9, 8.18.7.
+
+  Possible workarounds include:
+  * switching the order of the grouping keys (eg. `STATS ... BY keyword2, keyword1`, if the `keyword2` has a lower cardinality)
+  * reducing the grouping key cardinality, by filtering out values before STATS
+