[doc] ai_summary: say what the summary is for in the introduction

The introduction described the mechanism before the purpose.  It now
states what the plugin does, that it is meant for a local LLM server
speaking the OpenAI chat completions API, and -- the part that was
missing -- that the summary exists to condense the results the query
actually returned rather than to recite training data.

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

docs/dev/plugins/ai_summary.rst

@@ -10,15 +10,23 @@ AI Summary
    - :ref:`dev plugin`
    - :ref:`result types`
 
-The AI Summary plugin shows a generated answer above the search results.  The
-text comes from an LLM server the administrator runs; see :ref:`its
-configuration <settings ai_summary>` for how to set one up.
+The AI Summary plugin shows a generated answer above the ordinary search
+results, unless an engine has already answered the query directly -- with a
+Wikipedia infobox, for instance, or an instant answer.  It is meant to run
+against a local LLM server and speaks the `OpenAI chat completions API`_, so it
+works with `Ollama`_, Hugging Face TGI, LiteLLM, vLLM, llama.cpp and anything
+else that implements that specification.  See :ref:`its configuration <settings
+ai_summary>` for how to set one up.
 
-The interesting part of this plugin is *when* things happen.  Generating an
-answer takes seconds, and a search engine that waits seconds before painting
-anything is a broken search engine.  So the plugin never blocks the result
-page: it renders an empty box, and the browser fills that box afterwards from a
-second, streaming request.
+The purpose of the summary is not to tell you what the model memorised during
+training, but to summarise the up to date results your query actually returned.
+That is what the *grounding* setting does, and why it is enabled by default.
+
+Generating an answer takes seconds, and a search engine that waits seconds
+before painting anything is a broken search engine.  The summary is therefore
+produced asynchronously: the plugin renders an empty box, the result page is
+delivered immediately, and the browser fills that box from a second, streaming
+request.  The results below stay readable and scrollable the whole time.
 
 
 Request flow
@@ -122,6 +130,8 @@ The streaming response uses ``direct_passthrough``, so the generator must yield
 ``bytes`` -- werkzeug asserts on ``str``, and the Flask test client does not
 catch it.
 
+.. _Ollama: https://ollama.com/
+.. _OpenAI chat completions API: https://platform.openai.com/docs/api-reference/chat
 .. _SSE: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events
 .. _NDJSON: https://github.com/ndjson/ndjson-spec