bpo-41724: Explain when the conversion is not possible with detect_types enabled (GH-23855)

* Explain when the conversion is not possible with detect_types enabled

Author: sblondon

Doc/library/sqlite3.rst

@@ -198,7 +198,9 @@ Module functions and constants
 
    *detect_types* defaults to 0 (i. e. off, no type detection), you can set it to
    any combination of :const:`PARSE_DECLTYPES` and :const:`PARSE_COLNAMES` to turn
-   type detection on.
+   type detection on. Due to SQLite behaviour, types can't be detected for generated
+   fields (for example ``max(data)``), even when *detect_types* parameter is set. In
+   such case, the returned type is :class:`str`.
 
    By default, *check_same_thread* is :const:`True` and only the creating thread may
    use the connection. If set :const:`False`, the returned connection may be shared