[8.18] CCS and ES|QL rule reqs (#6640) (#6647)

* First draft

* fix title

* Tweaks

* List format

* Update docs/detections/rules-cross-cluster-search.asciidoc

Co-authored-by: Benjamin Ironside Goldstein <[email protected]>

---------

Co-authored-by: Benjamin Ironside Goldstein <[email protected]>
(cherry picked from commit cff400b)

Co-authored-by: Nastasha Solomon <[email protected]>

Author: mergify[bot]

docs/detections/rules-cross-cluster-search.asciidoc

@@ -3,6 +3,15 @@
 
 {ref}/modules-cross-cluster-search.html[Cross-cluster search] is an {es} feature that allows one cluster (the _local_ cluster) to query data in a separate cluster (the _remote_ cluster). {elastic-sec}'s detection rules can perform a cross-cluster search to query data in remote clusters.
 
+.Requirements
+[sidebar]
+--
+
+* To learn about the requirements for using cross-cluster search, refer to {ref}/modules-cross-cluster-search.html[Search across clusters].
+* Using cross-cluster search for {esql} rules requires an (https://www.elastic.co/pricing)[Enterprise subscription]. 
+
+--
+
 [discrete]
 [[set-up-ccs-rules]]
 === Set up cross-cluster search in detection rules