[9.2] [API DOCS]: Update Upgrade Assistant description (#244102) (#244750)

# Backport

This will backport the following commits from `main` to `9.2`:
- [[API DOCS]: Update Upgrade Assistant description
(#244102)](#244102)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

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Co-authored-by: Charlotte Hoblik <116336412+charlotte-hoblik@users.noreply.github.com>

Author: kibanamachine

oas_docs/output/kibana.yaml

@@ -358,7 +358,12 @@ tags:
       url: https://www.elastic.co/docs/deploy-manage/distributed-architecture/kibana-tasks-management
     name: task manager
     x-displayName: Task manager
-  - description: The assistant helps you prepare for the next major version of Elasticsearch.
+  - description: |
+      The Kibana Upgrade Assistant API helps you prepare for the next major Elasticsearch release.
+      > warn
+      > This is a Kibana REST API (not an Elasticsearch API) and requests must target your Kibana URL:
+      > * Self-managed URL pattern: `https://localhost:5601`
+      > * Elastic Cloud URL pattern: `https://your-deployment.kb.us-east-1.aws.elastic.cloud:9243`
     name: upgrade
     x-displayName: Upgrade assistant
   - description: Uptime APIs enable you to view and update uptime monitoring settings.

x-pack/platform/plugins/private/upgrade_assistant/docs/openapi/upgrade_apis.yaml

@@ -8,7 +8,12 @@ info:
     url: https://www.elastic.co/licensing/elastic-license
 tags:
   - name: upgrade
-    description: The assistant helps you prepare for the next major version of Elasticsearch.
+    description: |
+      The Kibana Upgrade Assistant API helps you prepare for the next major Elasticsearch release.
+      > warn
+      > This is a Kibana REST API (not an Elasticsearch API) and requests must target your Kibana URL:
+      > * Self-managed URL pattern: `https://localhost:5601`
+      > * Elastic Cloud URL pattern: `https://your-deployment.kb.us-east-1.aws.elastic.cloud:9243`
     x-displayName: Upgrade assistant
 servers:
   - url: /