Document the behavior of IM rules and multi-value indicator documents (…

…#4326)

(cherry picked from commit ed4d816)

docs/detections/rules-ui-create.asciidoc

@@ -177,6 +177,7 @@ NOTE: {elastic-sec} provides limited support for indicator match rules. See <<su
 
 . Go to *Rules* -> *Detection rules (SIEM)* -> *Create new rule*. The *Create new rule* page displays.
 . To create a rule that searches for events whose specified field value matches the specified indicator field value in the indicator index patterns, select *Indicator Match*, then fill in the following fields:
+
 .. *Source*: The individual index patterns or data view that specifies what data to search.
 .. *Custom query*: The query and filters used to retrieve the required results from
 the {elastic-sec} event indices. For example, if you want to match documents that only contain a `destination.ip` address field, add `destination.ip : *`.
@@ -192,9 +193,13 @@ 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 {elastic-sec} event
 indices.
 ** *Indicator index field*: The field used for comparing values in the indicator