[DOCS] Adds source index privileges required for Explain DFA API docs. (elastic#70978)

Author: szabosteve

docs/reference/ml/df-analytics/apis/explain-dfanalytics.asciidoc

@@ -30,8 +30,10 @@ If the {es} {security-features} are enabled, you must have the following
 privileges:
 
 * cluster: `monitor_ml`
+* source indices: `read`, `view_index_metadata`
 
-For more information, see <<security-privileges>> and {ml-docs-setup-privileges}.
+For more information, see <<security-privileges>> and 
+{ml-docs-setup-privileges}.
 
 
 [[ml-explain-dfanalytics-desc]]