[doc] ai_summary: document what grounding sends to the LLM server

Grounding is on by default, and it sends the top search results along
with the query.  Which data leaves the instance depends on where the LLM
server runs -- per the privacy-by-design directive the reader has to be
able to see that from the documentation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/admin/settings/settings_ai_summary.rst

@@ -38,6 +38,24 @@ The *AI Summary* tab of the preferences is only shown when the plugin is
 activated in ``settings.yml`` (``active: true``); an instance that does not
 offer AI summaries does not show the tab at all.
 
+.. _ai_summary grounding:
+
+Grounding
+=========
+
+With ``grounding`` (the default) the query **and the top search results**
+(title, URL and snippet of at most ``max_context_items`` results) are sent to
+the LLM server; without it only the query is sent and the model answers from
+its own knowledge.  Grounded answers are more accurate and more current, at
+the cost of a longer prompt.
+
+Which data leaves the SearXNG instance therefore depends on where the LLM
+server runs: with a server on localhost or in the local network nothing leaves
+the network, with a hosted server the query and the result snippets are sent
+to that provider.  This is why the plugin is not activated by default and why
+its documentation recommends a local LLM server.  Users can opt out of
+grounding in their preferences (``ai_summary_grounding``).
+
 .. attention::
 
    A user configurable server URL allows any user of the instance to make the