[8.9] [Known Issue] Add docs to describe a known issue/limitation of EQL rule cross-cluster search (#4813)

docs/detections/api/rules/rules-api-create.asciidoc

@@ -413,11 +413,16 @@ documents from the {es} index containing the threat values.
 context] array used to define the conditions for when alerts are created from
 events. Defaults to an empty array.
 
-|index |String[] |Indices on which the rule functions. Defaults to the
+|index |String[] a|Indices on which the rule functions. Defaults to the
 Security Solution indices defined on the {kib} Advanced Settings page
 (*Kibana* → *Stack Management* → *Advanced Settings* →
 `securitySolution:defaultIndex`).
 
+[NOTE]
+======
+Event correlation rules have a limitation that prevents them from querying multiple indices from different clusters (local and remote). To enable this, a user with the {ref}/built-in-roles.html[`superuser`] role must modify the EQL rules that are configured to use <<rules-cross-cluster-search,cross-cluster search>>. This updates the rule's API key to use `superuser` privileges and allows the rule to use cross-cluster search.
+======
+
 |risk_score_mapping |Object[] a|Overrides generated alerts' `risk_score` with
 a value from the source event:
 

docs/detections/rules-ui-create.asciidoc

@@ -209,6 +209,9 @@ IMPORTANT: Alerts created by threshold rules are synthetic alerts that do not re
 ==== Create an event correlation rule
 . To create an event correlation rule using EQL, select *Event Correlation*, then:
 .. Define which {es} indices or data view the rule searches when querying for events.
++
+NOTE: Event correlation rules have a limitation that prevents them from querying multiple indices from different clusters (local and remote). To enable this, a user with the {ref}/built-in-roles.html[`superuser`] role must modify the EQL rules that are configured to use <<rules-cross-cluster-search,cross-cluster search>>. This updates the rule's API key to use `superuser` privileges and allows the rule to use cross-cluster search.
+
 .. Write an {ref}/eql-syntax.html[EQL query] that searches for matching events or a series of matching events.
 +
 TIP: To find events that are missing in a sequence, use the {ref}/eql-syntax.html#eql-missing-events[missing events] syntax.