update retrieve_elastic_doc prompt to generate query in English (elastic#222303)

Closes
[elastic#233](elastic/obs-ai-team#233)
## Summary

Update `retrieve_elastic_doc` prompt to generate the query always in
English, as the documentation is available only in English.

Co-authored-by: Viduni Wickramarachchi <[email protected]>
Co-authored-by: Søren Louv-Jansen <[email protected]>

Author: 3 people

x-pack/solutions/observability/plugins/observability_ai_assistant_app/server/functions/documentation.ts

@@ -43,6 +43,7 @@ export async function registerDocumentationFunction({
         properties: {
           query: {
             description: `The query to use to retrieve documentation
+            Always write the query in English, as the documentation is available only in English.
             Examples:
             - "How to enable TLS for Elasticsearch?"
             - "What is Kibana Lens?"`,