ESQL: Add known issue for VALUES (8.17)

They are so so so slow that this is effectively a bad bug.

Author: nik9000

docs/reference/release-notes/8.17.2.asciidoc

@@ -48,4 +48,8 @@ Snapshot/Restore::
 Ingest Node::
 * Improve memory aspects of enrich cache {es-pull}120256[#120256] (issues: {es-issue}96050[#96050], {es-issue}120021[#120021])
 
+[discrete]
+[[known-issues-8.17.2]]
+=== Known issues
 
+* `VALUES` aggregate function can run for a long, long time when collecting many, many groups. Hundreds of thousands of groups can run on one thread for many minutes and millions of groups run for days. It is O(n^2^) with the number of groups. These will not respond to the tasks cancellation API the whole time. Fixed by {es-pull}#123073[#123073] and available in 8.16.5, 8.17.3, 8.18.0, and all releases after that.