Add information about the default sort mode (#40657)

docs/reference/search/request/sort.asciidoc

@@ -82,6 +82,10 @@ to. The `mode` option can have the following values:
 `median`:: Use the median of all values as sort value.  Only applicable
            for number based array fields.
 
+The default sort mode in the ascending sort order is `min` -- the lowest value
+is picked. The default sort mode in the descending order is `max` --
+the highest value is picked.
+
 ===== Sort mode example usage
 
 In the example below the field price has multiple prices per document.