Add applies_to label to logsdb message default sort setting (#137767)

This PR updates the docs for index.logsdb.default_sort_on_message_template
to indicate that it is a preview feature in stack release 9.3.

Author: jordan-powers

docs/reference/elasticsearch/mapping-reference/pattern-text.md

@@ -46,8 +46,18 @@ In both cases, all queries return a constant score of 1.0.
 
 ## Index sorting for improved compression
 The compression provided by `pattern_text` can be significantly improved if the index is sorted by the `template_id` field.
-This sorting is not applied by default, but can be enabled for the `message` field of LogsDB indices (assuming it is of type `pattern_text`) by setting the index setting `index.logsdb.default_sort_on_message_template` to `true`.
+
+
+### Default sorting for LogsDB
+```{applies_to}
+serverless: preview
+stack: preview 9.3
+```
+This index sorting is not applied by default, but can be enabled for the `message` field of LogsDB indices (assuming it is of type `pattern_text`) by setting the index setting `index.logsdb.default_sort_on_message_template` to `true`.
 This will cause the index to be sorted by `host.name` (if present), then `message.template_id`, and finally by `@timestamp`.
+
+
+### Custom sorting
 If the index is not LogsDB or the `pattern_text` field is named something other than `message`, index sorting can still be manually applied as shown in the following example.
 
 ```console
@@ -73,6 +83,7 @@ PUT logs
 ```
 
 
+
 ## Parameters for pattern text fields [pattern-text-params]
 
 The following mapping parameters are accepted: