Add info about the defaults values of index.sort.* settings (elastic#142877)

Fixes [elastic#56](elastic/docs-content-internal#56)

Author: yetanothertw

docs/reference/elasticsearch/index-settings/sorting.md

@@ -80,16 +80,20 @@ $$$index-sort-order$$$
 `index.sort.order` {applies_to}`serverless: all`
 :   The sort order to use for each field. The order option can have the following values:
 
-    * `asc`:  For ascending order
+    * `asc`:  For ascending order.
     * `desc`: For descending order.
 
+    When `index.sort.field` is set but `index.sort.order` is not explicitly provided, each field defaults to `asc`.
+
 $$$index-sort-mode$$$
 `index.sort.mode` {applies_to}`serverless: all`
 :   Elasticsearch supports sorting by multi-valued fields. The mode option controls what value is picked to sort the document. The mode option can have the following values:
 
     * `min`: 	Pick the lowest value.
     * `max`: 	Pick the highest value.
 
+    When `index.sort.order` is set to `asc`, `index.sort.mode` defaults to `min`. If you set the order to `desc`, the mode defaults to `max`.
+
 
 $$$index-sort-missing$$$
 `index.sort.missing` {applies_to}`serverless: all`
@@ -98,6 +102,8 @@ $$$index-sort-missing$$$
     * `_last`: Documents without value for the field are sorted last.
     * `_first`: Documents without value for the field are sorted first.
 
+    Documents missing the sort field are always sorted last by default, regardless of sort order.
+
 
 ::::{warning}
 Index sorting can be defined only once at index creation. It is not allowed to add or update a sort on an existing index. Index sorting also has a cost in terms of indexing throughput since documents must be sorted at flush and merge time. You should test the impact on your application before activating this feature.