ES|QL: Add Known Issues note about STATS #130644

Author: luigidellaquila

docs/reference/release-notes/8.16.0.asciidoc

@@ -517,3 +517,12 @@ 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 STATS command may lead to wrong results. The bug only happens in a very specific case, ie. with `STATS ... BY keyword1, keyword2`:
+exactly two grouping fields, both keywords, where the first field has high cardinality (more than 65k distinct values).
+
+  The issue is described in detail in [this issue](https://github.com/elastic/elasticsearch/issues/130644).
+
+  Possible work-arounds are:
+  * switching the order of the grouping keys (eg. `STATS ... BY keyword2, keyword1`, if the `keyword2` has a lower cardinality)
+  * reducing the grouping key cardinality, filtering out values before STATS.