[DOCS] Add wait_for_completion parm to execute enrich policy API docs (#48077)

Author: jrodewig

docs/reference/ingest/apis/enrich/execute-enrich-policy.asciidoc

@@ -100,4 +100,12 @@ it may take a while to return a response.
 
 `<enrich-policy>`::
 (Required, string)
-Enrich policy to execute.
+Enrich policy to execute.
+
+[[execute-enrich-policy-api-request-body]]
+==== {api-request-body-title}
+
+`wait_for_completion`::
+(Required, boolean)
+If `true`, the request blocks other enrich policy execution requests until
+complete. Defaults to `true`.