Note to configuration docs on number of threads

This commit adds a note to the configuration docs regarding the number
of threads necessary for the Elasticsearch user.

Relates #17003

docs/reference/setup/configuration.asciidoc

@@ -43,6 +43,13 @@ using the <<cluster-nodes-info>> API, with:
 curl localhost:9200/_nodes/stats/process?pretty
 --------------------------------------------------
 
+[float]
+[[max-number-of-threads]]
+==== Number of threads
+
+Make sure that the number of threads that the Elasticsearch user can
+create is at least 2048.
+
 [float]
 [[vm-max-map-count]]
 ==== Virtual memory