[DOCS] Clarifies retention policy for transforms (#89685)

docs/reference/rest-api/common-parms.asciidoc

@@ -1003,11 +1003,14 @@ criteria is deleted from the destination index.
 end::transform-retention[]
 
 tag::transform-retention-time[]
-Specifies that the {transform} uses a time field to set the retention policy.
+Specifies that the {transform} uses a time field to set the retention policy. 
+Data is deleted if `time.field` for the retention policy exists and contains 
+data older than `max.age`.
 end::transform-retention-time[]
 
 tag::transform-retention-time-field[]
-The date field that is used to calculate the age of the document.
+The date field that is used to calculate the age of the document. Set 
+`time.field` to an existing date field.
 end::transform-retention-time-field[]
 
 tag::transform-retention-time-max-age[]