[8.2] Document the behavior of IM rules and multi-value indicator doc…

…uments (backport #4326) (#4405)

Co-authored-by: Nastasha Solomon <79124755+nastasha-solomon@users.noreply.github.com>
Co-authored-by: nastasha.solomon <nastasha.solomon@elastic.co>

docs/detections/rules-ui-create.asciidoc

@@ -226,10 +226,14 @@ IMPORTANT: Data in indicator indices must be <<ecs-compliant-reqs, ECS compatibl
 +
 .. *Indicator index query*: The query and filters used to filter the fields from
 the indicator index patterns. The default query `@timestamp > "now-30d/d"` searches specified indicator indices for indicators ingested during the past 30 days and rounds the start time down to the nearest day (resolves to UTC `00:00:00`).
-.. *Indicator mapping*: Compares the values of the specified event and indicator field
-values. When the field values are identical, an alert is generated. To define
+.. *Indicator mapping*: Compares the values of the specified event and indicator fields, and generates an alert if the values are identical.
++
+NOTE: Only single-value fields are supported.
++
+To define
 which field values are compared from the indices add the following:
-** *Field*: The field used for comparing values in the {es-sec} event
+
+** *Field*: The field used for comparing values in the {elastic-sec} event
 indices.
 ** *Indicator index field*: The field used for comparing values in the indicator
 indices.