(Doc+) Add typed_keys parameter set as true

### Description
This PR Add `typed_keys` parameter set as `true`:
- a `JsonException` that occurs when using aggregations with `SearchTemplateAsync` because the mandatory `typed_keys=true` parameter is undocumented.
- This change improves the developer experience by preventing user confusion and saving significant debugging time when using search templates.

### Related issues
- #7605 (comment)
- elastic/sdh-search#1648 (comment)

Author: jihyunjeongme

docs/usage/aggregations.asciidoc

@@ -3,6 +3,22 @@
 
 This page demonstrates how to use aggregations.
 
+[discrete]
+=== Using Aggregations with Search Templates
+
+[IMPORTANT]
+.A Note on `typed_keys` and `SearchTemplateAsync`
+====
+When using aggregations with the `SearchTemplateAsync` API, it is **mandatory** to include the `typed_keys: true` parameter in your request.
+
+Unlike the `SearchAsync` API, which adds the `typed_keys=true` parameter to requests automatically, the `SearchTemplateAsync` API cannot do so due to the flexible nature of search templates.
+
+If this parameter is omitted, the client's deserializer will fail with a `JsonException` as it cannot determine the aggregation types from the response.
+====
+
+To correctly receive aggregations from a search template, you must explicitly include `typed_keys: true` in the parameters you provide to the `SearchTemplateAsync` call.
+
+
 [discrete]
 === Top-level aggreggation