[Obs AI Assistant] Update information about the Kibana tool (#3968)

Closes elastic/kibana#241061

The Kibana tool requires the `server.publicBaseUrl` to be set in
`kibana.yml` after the merging this fix -
elastic/kibana#240349

This PR adds this requirement to the Observability AI Assistant
documentation.

---------

Co-authored-by: Mike Birnstiehl <[email protected]>

Author: viduni94

solutions/observability/observability-ai-assistant.md

@@ -335,6 +335,11 @@ Main functions:
 `kibana`
 :   Call {{kib}} APIs on your behalf.
 
+::::::{important}
+:applies_to: self:
+For self‑managed deployments, you must configure [`server.publicBaseUrl`](kibana://reference/configuration-reference/general-settings.md#server-publicbaseurl) in your `kibana.yml` to use the `kibana` function.
+::::::
+
 `query`
 :   Generate, execute, and visualize queries based on your request.