[DOCS] Adds description to the preview data frame transform API (#43745)

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

@@ -30,6 +30,15 @@ Previews a {dataframe-transform}.
 {stack-ov}/security-privileges.html[Security privileges] and
 {stack-ov}/built-in-roles.html[Built-in roles].
 
+[discrete]
+[[preview-data-frame-transform-desc]]
+==== {api-description-title}
+
+This API generates a preview of the results that you will get when you run the
+<<put-data-frame-transform,create {dataframe-transforms} API>> with the same
+configuration. It returns a maximum of 100 results. The calculations are based
+on all the current data in the source index. 
+
 [discrete]
 [[preview-data-frame-transform-request-body]]
 ==== {api-request-body-title}
@@ -40,6 +49,15 @@ Previews a {dataframe-transform}.
 `pivot` (Required)::
   (object) Defines the pivot function `group by` fields and the aggregation to
   reduce the data. See <<data-frame-transform-pivot>>.
+  
+[discrete]
+[[preview-data-frame-transform-response]]
+==== {api-response-body-title}
+
+`preview`::
+  (array) An array of documents. In particular, they are the JSON
+  representation of the documents that would be created in the destination index
+  by the {dataframe-transform}. 
 
 [discrete]
 ==== {api-examples-title}