Document that rejected write PRAGMAs in db.query() still take effect

db.query() promises that a statement rejected with ValueError is rolled
back and has no effect. PRAGMA statements are the exception: some of them
refuse to run inside a transaction, so they execute outside the savepoint
guard - a row-less PRAGMA such as "PRAGMA user_version = 5" therefore
takes effect despite the ValueError. Documenting this as a known
limitation rather than fixing it, since a fix would need a hardcoded list
of row-returning PRAGMAs.

Refs https://github.com/simonw/sqlite-utils/issues/769#issuecomment-4900034150

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

docs/python-api.rst

@@ -233,6 +233,8 @@ The SQL query is executed as soon as ``db.query()`` is called. The resulting row
 
 ``db.query()`` can only be used with SQL that returns rows. Passing a statement that returns no rows - an ``INSERT`` or ``UPDATE`` without a ``RETURNING`` clause, for example - will raise a ``ValueError``. The rejected statement is rolled back, so it has no effect on the database. Use :ref:`db.execute() <python_api_execute>` for those statements instead.
 
+There is one exception to the rolled-back guarantee: a ``PRAGMA`` statement that returns no rows, such as ``PRAGMA user_version = 5``, still raises a ``ValueError`` but will already have taken effect. Some PRAGMA statements refuse to run inside a transaction, so PRAGMAs are executed outside the savepoint that is used to roll back other rejected statements. Use ``db.execute()`` for PRAGMA statements that do not return rows.
+
 If a query returns more than one column with the same name - a join between two tables that share column names, for example - later occurrences are renamed with a numeric suffix, so every value is included in the dictionary:
 
 .. code-block:: python