bpo-41724: Explain when the conversion is not possible with detect_types enabled (GH-23855) (GH-23863)

* Explain when the conversion is not possible with detect_types enabled
(cherry picked from commit 09a36cd)

Co-authored-by: sblondon <sblondon@users.noreply.github.com>

Author: miss-islington

Doc/library/sqlite3.rst

@@ -197,7 +197,9 @@ 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