[DOCS] Correct over field usage for freq rare function (elastic/x-pack-elasticsearch#3435)
Original commit: elastic/x-pack-elasticsearch@9fab3a19a7e612d5b024ffa48fb11688e56ee5c7
@@ -96,7 +96,7 @@ values occur.
This function supports the following properties:
* `by_field_name` (required)
-* `over_field_name` (optional)
+* `over_field_name` (required)
* `partition_field_name` (optional)
For more information about those properties, see
