[DOCS] Adds frequency option to data frame transform resource (#44177)

docs/reference/data-frames/apis/get-transform.asciidoc

@@ -143,6 +143,7 @@ The API returns the following results:
       "dest" : {
         "index" : "kibana_sample_data_ecommerce_transform"
       },
+      "frequency": "1m",
       "pivot" : {
         "group_by" : {
           "customer_id" : {

docs/reference/data-frames/apis/transformresource.asciidoc

@@ -19,6 +19,11 @@ For more information, see
   (object) The destination for the {dataframe-transform}. See
   <<data-frame-transform-dest>>.
   
+`frequency`::
+  (time units) The interval between checks for changes in the source indices
+  when the {dataframe-transform} is running continuously. The minimum value is
+  `1s` and the maximum is `1h`. The default value is `1m`.
+
 `id`::
   (string) A unique identifier for the {dataframe-transform}.